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. 2010 Dec 14;17(8):2029–2040. doi: 10.1007/s00894-010-0913-3

Extrapolation of water and formaldehyde harmonic and anharmonic frequencies to the B3LYP/CBS limit using polarization consistent basis sets

Aneta Buczek 1, Teobald Kupka 1, Małgorzata A Broda 1,
PMCID: PMC3143340  PMID: 21153906

Abstract

The harmonic and anharmonic frequencies of fundamental vibrations in formaldehyde and water were successfully estimated using the B3LYP Kohn-Sham limit. The results obtained with polarization- and correlation-consistent basis sets were fitted with a two-parameter formula. Anharmonic corrections were obtained by a second order perturbation treatment (PT2). We compared the performance of the PT2 scheme on the two title molecules using SCF, MP2 and DFT (BLYP, B3LYP, PBE and B3PW91 functionals) methods combined with polarization consistent pc-n (n = 0, 1, 2, 3, 4) basis sets, Dunning’s basis sets (aug)-cc-pVXZ where X = D, T, Q, 5, 6 and Pople’s basis sets up to 6-311++G(3df,2pd). The influence of SCF convergence level and density grid size on the root mean square of harmonic and anharmonic frequency deviations from experimental values was tested. The wavenumber of formaldehyde CH2 anharmonic asymmetric stretching mode is very sensitive to grid size for large basis sets; this effect is not observed for harmonic modes. BLYP-calculated anharmonic frequencies consistently underestimate observed wavenumbers. On the basis of formaldehyde anharmonic frequencies, we show that increasing the Pople basis set size does not always lead to improved agreement between anharmonic frequencies and experimental values.

Figure.

Figure

Sensitivity of water B3LYP calculated harmonic and anharmonic v s(OH) frequencies on selected Pople and polarization consistent basis sets size. The results for pc-n basis sets were fitted with two parameter formula and the CBS(2,3,4) estimated

Electronic supplementary material

The online version of this article (doi:10.1007/s00894-010-0913-3) contains supplementary material, which is available to authorized users.

Keywords: Harmonic, Anharmonic, Complete basis set limit, IR and Raman theoretical spectra

Introduction

Apart from nuclear magnetic resonance (NMR) techniques, infrared (IR) and Raman vibrational spectroscopies are the two analytical techniques most often used for chemical characterization of small, medium and large size chemicals and their mixtures. In addition, changes in vibrational frequencies are used to study strong and weak inter- and intramolecular interactions (hydrogen bonds, association and aromatic stacking) and chemical reactions. Accurate knowledge of spectrum-molecular structure relationships is important in DNA and enzymatic studies, as well as in biochemistry and pharmacology. It is therefore obvious that theoretical predictions should provide reliable frequencies and band intensities in order to support analysis of observed vibrational spectra.

Vibrational frequencies (wavenumbers) predicted theoretically at self-consistent field (SCF), density functional theory (DFT) and second order Møller–Plesset (MP2) levels of calculations are overestimated due to anharmonicity effects [1]. This effect is most severe (over 10%) in the case of SCF predicted C–H, N–H and O–H stretching vibrations. To date, almost 4,000 papers have cited the first study in which a simple remedy was proposed to cure the deficiency in this theory by the use of scaling factors [2]. Thus, scaled theoretical wavenumbers [24] are used to reliably compare predicted IR and Raman spectral numbers with experimental data (we will not discuss scaling of individual force constants here). The uncertainties of combinations of 40 methods and basis sets have been studied [5]. Estimation of empirical scaling factors from analysis of numerous compounds and their fundamental vibrations is very tedious work [2]. Obviously, there are still some inherent errors in the proposed scaling factors. For example, Hartree-Fock (HF)-scaled frequencies show less uncertainty than the corresponding MP2 frequencies [5, 6]. The most often used approach is based on a single scaling factor, while more sophisticated studies use individual scaling of low and high frequencies, as well as scaling for individual modes [e.g., ν(C = O), ν(OH), ν(CH)].

Structural and vibrational parameters predicted by theoretical methods depend on the level of theory, inclusion of correlation effects, and the completeness of the one-electron basis set used. For practical reasons, DFT [79] including some degree of electron correlation is the best compromise between accuracy and size of the molecular system studied, and B3LYP is a typical choice of density functional.

Among the high number of basis sets available, the so-called Pople sets, though fairly old, are robust and relatively small. Sometimes they reproduce experimental parameters very well. However, there is no regular change in energy toward the complete basis set limit (CBS) calculated using Pople basis sets. Dunning and coworkers [1013] utilized the idea of smooth and regular converging energy toward the CBS for constructing correlation-consistent basis set hierarchies [(aug)-cc-pVXZ, where X = D, T, Q, 5 and 6]. Thus, the CBS energy, and some other structural and spectral parameters were estimated using simple 2- and 3-parameter formulas. Obviously, the most accurate results were obtained for larger X (Q, 5 and 6). Later, Jensen [1419], and also Jorge [20] designed other families of converging basis sets. In particular, Jensen’s polarized-consistent basis sets pc-n, where n = 0, 1, 2, 3 and 4 seem to converge faster than with Dunning’s sets, while reproducing the calculated parameters in the SCF, DFT, MP2 and coupled cluster, singles and doubles with triples treated approximately [CCSD(T)] basis set limits [21, 22].

Several benchmark studies have been published recently on coupled cluster (CC) predicted geometry and vibrational frequencies of selected small molecules using the correlation-consistent basis sets [2325]. In fact, the frequencies of water [25, 26] and formaldehyde [25] have been very well reproduced using high level calculations. Unfortunately, CC methodology is prohibitively expensive for larger molecules. However, the new, less popular but more affordable pc-n basis sets were not employed in these benchmark tests. Besides, there is an open question about Kohn-Sham limiting values of vibrational frequencies obtained using harmonic and anharmonic models.

In this study we will address the problem of the accuracy of calculated harmonic and anharmonic vibrational frequencies for water and formamide in the gas phase using Pople vs Jensen’s and Dunning’s basis sets, and the convergence of individual results toward B3LYP CBS. In addition, the accuracy of the density grid in calculated harmonic and anharmonic frequencies will be tested. Water and formaldehyde were selected as simple model molecules for our study as their harmonic and anharmonic frequencies in the gas phase are well known. Several works comparing the theoretical and experimental vibrational spectra of these molecules have been published [2529]. Moreover, their structural and vibrational parameters are modified by intermolecular interactions, including solute–solvent interactions. Thus, the conclusions of current study will aid further detailed studies on amides and small polypeptides in the gas phase and solution.

Therefore, in this work, we will test the performance of a typical, easy to compute harmonic model, and a more computationally demanding anharmonic method. Both methods are available in Gaussian 09 [30] and other software packages. We will also apply an empirical (single or global) scaling factor to harmonic frequencies and compare the results obtained with experimental and previously reported wavenumbers.

Theoretical calculations

All calculations were performed using the Gaussian 09 program [30] and some results were confirmed using Gaussian 03 [31].

Basis sets and density functionals

Pople’s 3-21G, 6-31G, 6-31G*, 6-311++G** and 6-311++G(3df,2pd), Jensen’s pc-n polarized-consistent, and Dunning’s (aug)-cc-pVXZ basis sets were used. The efficient B3LYP density functional was selected and, for comparison purposes, some calculations were also performed at restricted HF (RHF) and MP2 levels. In addition, several other common DFT methods were selected (BLYP, B3PW91 and PBE). The pc-n basis sets were downloaded from EMSL [32].

Geometry

Fully optimized geometries of water and formaldehyde in the gas phase were obtained using default and very tight convergence criteria for each method and basis set selected. All positive harmonic vibration frequencies were obtained ensuring ground state structures.

Harmonic and anharmonic vibration calculations

The calculations were carried out in the gas phase (vacuum) using the VPT2 method as implemented by Barone [33, 34] in the Gaussian program package. In several cases, the finest DFT integration grid was selected by using in the command line SCF=tight and Int(Grid=150590) instead of Int(Grid=ULTRAFINE) keyword. The use of such a fine grid is critical in the case of indirect spin–spin coupling constant calculations with tailored basis sets [35, 36].

CBS calculations

The harmonic and anharmonic frequencies, Y(x), were calculated using polarization-consistent pc-n basis sets, where n = 0, 1, 2, 3 and 4, and the correlation-consistent (aug)-cc-pVXZ basis sets, where X = D, T, Q, 5 and 6, and subsequently extrapolated to the B3LYP CBS limit, Y(∞), by fitting the results to two-parameter functions [37]:

graphic file with name M1.gif 1

The extrapolated value Y(∞) corresponds to the best estimate of the predicted property for infinite zeta (or cardinal number “X”), where A and Y(∞) are fitted parameters. In the case of Jensen’s pc-n basis sets, X = n + 2 was assumed for graphical fitting purposes only [21, 38]. All fittings were performed with a two-parameter formula (Eq. 1), in several cases enabling exact fitting of only two data points. Since smaller values of “X” and “n” yield results (frequencies in this study) that are more corrupted by errors due to basis set imperfections, the CBS values are often estimated using higher cardinal numbers. For example, CBS(4,5,6) indicates estimation using X = Q, 5 and 6, or n = 2, 3 and 4, respectively.

Scaling factors

Single scaling factors were used for low and high frequencies. Three fundamental studies [24] on scaling factors are used in frequency and zero-point vibrational (ZPV) energy calculations. Evaluation of scaling factors is very laborious work and, therefore, despite the presence of myriad methods and basis sets, only a few scaling factors are available in the literature. In particular, scaling of results obtained with the recently introduced Jensen’s basis sets and very large Dunning’s basis sets is lacking. Thus, in several cases we arbitrary used values taken from similar basis sets. For the convenience of the reader, all the scaling factors used in our work are collected in one table (Table S1 in the electronic supplementary material).

Results and discussion

The B3LYP-calculated harmonic and anharmonic frequencies of water modes as a function of selected Pople and Jensen basis set size are shown in Fig. 1. For δ(HOH) mode, the wavenumbers predicted with Pople basis sets behave irregularly, and an increase in the basis set size (compare 6-31G and 6-31G*) does not lead to better prediction of this water vibration. On the other hand, the results obtained with Jensen basis sets change more regularly. Thus, we used Eq. 1 to fit the results of both harmonic and anharmonic frequencies for n = 2, 3 and 4 toward the basis set limit. The limiting values [CBS(harm) and CBS(anharm)] are shown in Fig. 1 as straight dashed lines and compared with experimentally observed results in the gas phase (straight solid line). Usually, we observed a significantly lower sensitivity of wavenumber to the size and completeness of pc-n basis set hierarchy than with the Pople basis sets. Moreover, one can see a significantly smaller deviation from experimental values for the estimated CBS anharmonic with respect to harmonic frequencies. For example, for the water OH asymmetric stretch mode these values are −34 vs 143 cm−1, respectively (Fig. 1). B3LYP-predicted formaldehyde vibrational modes show a similar dependence on basis set type and size (Fig. 2).

Fig. 1.

Fig. 1

Sensitivity of water B3LYP-calculated harmonic and anharmonic frequencies on selected Pople and polarization consistent basis sets size. The results for pc-n basis sets were fitted with Eq. 1 and the complete basis set limit (CBS) (2,3,4) estimated

Fig. 2.

Fig. 2

Sensitivity of formaldehyde B3LYP-calculated harmonic and anharmonic frequencies on selected Pople and polarization consistent basis sets size. The results for pc-n basis sets were fitted with Eq. 1 and the CBS(2,3,4) estimated

One might expect that, in the case of numerical calculations of anharmonic frequencies, the quality of the results could be influenced by the accuracy of the density grid, as in the case of the indirect spin–spin coupling constant [36, 39]. Detailed analysis of water and formaldehyde B3LYP frequency deviation from experimental values [40, 41], calculated with Pople and polarization-consistent basis sets, is shown in Tables 1 and 2, respectively. Both harmonic and anharmonic deviations of water individual stretching and deformation modes are compared with deviations from simple scaling of harmonic values for different basis sets. In addition, as some general measure of calculation accuracy, the standard root mean square (RMS) deviation values are shown. The top of Table 1 gathers the results obtained for default optimization and frequency conditions (keywords OPT, Freq=anharm), and compare them with results calculated using a very accurate density grid [keywords OPT=tight, Freq=anharm, SCF=tight, INT(GRID=150590)]. Thus, the upper half of Table 1 lists results for selected Pople basis sets, and the bottom half the corresponding values obtained with Jensen’s basis sets and the final CBS values. Similar results obtained for formaldehyde are presented in the same way in Table 2. First, it is evident from Table 1 that there is no impact of grid size on the accuracy of water frequency prediction for either Pople or Jensen’s basis sets. However, in the case of high frequency formaldehyde anharmonic vibrations [νasym (CH2) in Table 2], grid size has a significant impact on the two largest Pople [6-311++G** and 6-311++G(3df,2pd)] and Jensen’s basis sets (n = 1, 2, 3 and 4 as well as CBS). Thus, a more accurate density grid is important for improving formaldehyde anharmonic frequency accuracy. On the contrary, formaldehyde harmonic frequencies do not change upon changing grid size.

Table 1.

Deviations of water B3LYP harmonic (Δharm), anharmonic (Δanh) and scaled harmonic (Δscal) frequencies (cm−1) calculated with selected Pople and Jensen’s basis sets from experimental values

3-21G 6-31G 6-31G* 6-31+G** 6-311++G** 6-311++G(3df,2pd)
Mode exp.a Δharm Δanh Δscal Δharm Δanh Δscal Δharm Δanh Δscal Δharm Δanh Δscal Δharm Δanh Δscal Δharm Δanh Δscal
Computation criteria: opt freq=anharm
νas(OH) 3756 −200 −373 −338 25 −173 −121 93 −102 −56 175 −6 37 166 −18 43 169 −18 46
νs(OH) 3657 −242 −351 −374 −41 −215 −181 70 −102 −74 152 −13 18 160 −12 40 168 −6 48
δ(HOH) 1595 98 125 32 24 −31 −39 118 64 52 8 −41 −48 8 −39 −43 33 −22 −19
RMS 190 304 292 31 160 127 96 91 61 134 25 37 133 26 42 139 17 40
Computation criteria: opt=tight scf=tight INT(Grid = 150590) freq=anharm
νas(OH) 3756 −201 −368 −338 27 −174 −120 94 −100 −55 176 −12 38 166 −19 43 169 −19 46
νs(OH) 3657 −242 −345 −374 −40 −217 −180 71 −102 −73 153 −20 19 160 −13 40 168 −8 48
δ(HOH) 1595 98 127 32 24 −34 −39 118 68 52 8 −43 −48 8 −39 −43 33 −22 −18
RMS 190 300 292 31 162 127 96 91 61 135 28 37 133 26 42 139 17 40
pc-0 pc-1 pc-2 pc-3 pc-4 CBSb
Mode exp. Δharm Δanh Δscal Δharm Δanh Δscal Δharm Δanh Δscal Δharm Δanh Δscal Δharm Δanh Δscal Δharm Δanh Δscal
Computation criteria: opt freq=anharm
νas(OH) 3756 50 −157 −61 126 −70 6 159 −24 38 153 −27 55 153 −27 55 144 −31 46
νs(OH) 3657 −37 −224 −143 110 −74 −6 154 −15 36 150 −16 55 150 −16 55 143 −18 49
δ(HOH) 1595 −30 −92 −75 33 −22 −17 30 −23 −20 34 −18 −7 34 −18 −6 40 −11 −1
RMS 40 167 99 99 60 11 129 21 33 125 21 45 125 21 45 119 22 39
Computation criteria: opt=tight scf=tight INT(Grid = 150590) freq=anharm
νas(OH) 3756 54 −135 −57 126 −67 6 159 −28 38 153 −30 56 153 −30 55 143 −34 46
νs(OH) 3657 −34 −203 −140 110 −71 −7 155 −19 37 150 −20 55 150 −20 55 143 −21 49
δ(HOH) 1595 −32 −92 −78 33 −19 −17 30 −23 −20 34 −19 −7 34 −18 −6 40 −12 0
RMS 41 151 98 98 58 11 129 23 33 125 23 45 125 23 45 119 24 39

aFrom [40]

bEstimated for pc-n where n = 2,3,4 using Eq. 1

Table 2.

Deviations of formaldehyde harmonic (Δharm), anharmonic (Δanh) and scaled harmonic (Δscal) frequencies (cm−1) calculated with selected Pople and Jensen’s basis sets from the experimental values

3-21G 6-31G 6-31G* 6-31+G** 6-311++G** 6-311++G(3df,2pd)
Mode exp.a Δharm Δanh Δscal Δharm Δanh Δscal Δharm Δanh Δscal Δharm Δanh Δscal Δharm Δanh Δscal Δharm Δanh Δscal
Computation criteria: opt freq=anharm
νas(CH2) 2843 114 −139 0 192 −23 75 125 −129 11 137 −12 32 99 −167 7 99 −165 6
νs(CH2) 2782 129 −60 16 182 9 67 135 −31 22 133 −31 30 102 −60 12 102 −58 11
ν(CO) 1745 15 −18 −53 3 −27 −65 105 78 33 74 48 10 70 44 13 77 50 19
σ(CH2) 1500 76 42 15 60 29 0 63 29 2 37 3 −17 31 −1 −17 32 −1 −16
ρ(CH2) 1250 31 4 −19 24 2 −25 30 9 −20 13 −8 −32 10 −10 −30 16 −5 −23
τ(CH2) 1167 53 26 6 36 16 −11 31 12 −15 26 8 −16 35 14 −3 34 16 −4
RMS 81 65 25 112 20 50 92 64 20 86 24 24 68 75 16 69 74 15
Computation criteria: opt=tight scf=tight INT(Grid = 150590) freq=anharm
νas(CH2) 2843 113 −132 −2 190 5 72 125 −124 10 134 −9 29 98 −29 6 95 −29 3
νs(CH2) 2782 128 −52 15 179 16 64 134 −23 22 130 −25 27 101 −50 11 99 −51 8
ν(CO) 1745 15 −17 −53 2 −28 −65 105 78 33 74 47 10 69 44 12 76 49 18
σ(CH2) 1500 76 43 15 60 28 −1 63 29 2 37 3 −17 31 −1 −17 33 −1 −16
ρ(CH2) 1250 31 6 −19 25 2 −24 30 9 −20 13 −8 −32 10 −9 −30 17 −5 −23
τ(CH2) 1167 52 29 5 36 16 −11 31 12 −15 27 8 −15 35 14 −3 35 17 −3
RMS 80 62 25 111 19 49 92 62 20 84 23 23 67 30 16 67 32 14
pc-0 pc-1 pc-2 pc-3 pc-4 CBSb
Mode exp. Δharm Δanh Δscal Δharm Δanh Δscal Δharm Δanh Δscal Δharm Δanh Δscal Δharm Δanh Δscal Δharm Δanh Δscal
Computation criteria: opt freq=anharm
νas(CH2) 2843 176 −69 88 108 −162 17 102 −163 11 96 −165 23 96 −166 23 87 −170 14
νs(CH2) 2782 198 6 111 110 −58 21 105 −58 16 101 −59 29 101 −60 29 94 −63 23
ν(CO) 1745 −28 −60 −78 87 60 30 72 45 16 72 45 26 72 45 26 71 44 26
σ(CH2) 1500 30 −4 −15 32 −1 −15 34 1 −13 33 0 −5 33 −1 −6 30 −3 −8
ρ(CH2) 1250 −21 −39 −56 15 −5 −24 18 −4 −21 18 −4 −14 18 −4 −14 18 −4 −14
τ(CH2) 1167 13 −5 −21 22 5 −15 37 17 0 34 16 4 33 15 3 27 11 −2
RMS 110 41 71 74 74 21 70 73 14 67 74 20 67 74 19 63 76 17
Computation criteria: opt=tight scf=tight INT(Grid = 150590) freq=anharm
νas(CH2) 2843 173 −59 85 107 −25 16 101 −26 10 95 −29 22 95 −29 22 87 −33 14
νs(CH2) 2782 196 18 109 109 −48 20 103 −49 14 99 −51 28 99 −51 27 93 −54 22
ν(CO) 1745 −28 −60 −78 87 61 30 71 45 15 71 45 26 71 44 26 70 44 25
σ(CH2) 1500 30 −3 −15 32 −1 −15 34 1 −13 33 −1 −5 32 −1 −6 30 −4 −8
ρ(CH2) 1250 −21 −38 −57 15 −5 −24 18 −4 −21 18 −4 −13 18 −4 −14 18 −4 −14
τ(CH2) 1167 14 −3 −21 22 4 −14 37 18 0 33 15 4 33 15 3 27 11 −3
RMS 108 38 70 74 33 21 69 30 14 66 31 19 66 31 19 62 32 16

aFrom [41]

bEstimated for pc-n where n = 2,3,4 using Eq. 1

There is no clear dependence of Pople basis set size on RMS deviations of harmonic and anharmonic frequencies. For example, the 6-31G basis set predicts water harmonic frequencies relatively well compared to anharmonic ones. In contrast, the same basis set (6-31G) gives the opposite result in the case of formaldehyde. Thus, we should treat such behavior as the result of accidental error cancellation. In other words, vibrational analysis using small basis sets is unreliable due to basis set incompleteness. Larger Pople basis sets are associated with an improvement in prediction of water anharmonic frequencies. Thus, for the 6-311++G(3df,2pd) basis set, corresponding anharmonic and harmonic RMS deviations of 17 vs 139 cm−1 are observed. This is also clearly visible in Fig. 1. In the case of Jensen’s basis set, starting from n = 2, water anharmonic frequencies are predicted significantly better than harmonic frequencies (RMS deviations of 23 vs 129 cm−1 for pc-2). Moreover, the RMS values for anharmonic water frequencies predicted with Pople basis sets [other than 6-311++G(3df,2pd)] are larger than with the pc-n basis set.

The use of simple harmonic frequency scaling leads to fairly accurate water wavenumbers. The accuracy of scaled water wavenumbers is similar to the anharmonic results for the studied Pople and Jensen’s basis sets (Table 1), and, for formaldehyde, scaled harmonic frequencies are often even closer to experimental values than the anharmonic frequencies (Table 2).

Next, water and formaldehyde harmonic and anharmonic wavenumbers were calculated with Dunning’s cc-pVXZ and aug-cc-pVXZ basis sets. The results were very similar to those obtained earlier with Jensen’s basis sets (see Figs. S1S4 in the electronic supplementary material), and the corresponding deviations from experimental values are listed in Tables S2 and S3. Similarly to the results in Table 1, there is no dependence on grid size of water frequencies predicted with both Dunning’s basis set series (Table S2). However, in the case of formaldehyde, similarly to results obtained with Jensen’s basis set family (Table 2), the improvement in grid size used in conjunction with larger Dunning’s basis sets (cc-pVXZ for X = 5 and 6, and aug-cc-pVXZ for X = T, Q and 5) leads to an improvement in RMS of anharmonic frequencies of more than twofold, due mainly to a better description of CH2 asymmetric stretching. Moreover, in all cases the scaled harmonic frequencies for formaldehyde are significantly closer to experimental values than the corresponding anharmonic values (Table S3), and are comparable for water (Table S2).

The CBS values obtained with Jensen’s and Dunning’s basis set families are very similar for both molecules. However, it is important to note that Jensen’s basis sets allow significantly faster calculations than Dunning basis sets. The dependence of CPU time necessary for VPT2 calculations with pc-n, cc-pVXZ and aug-cc-pVXZ basis sets in the case of formaldehyde is presented in Fig. 3. For example, the CPU time for formaldehyde anharmonic calculations using cc-pV6Z and pc-4 basis sets with the same computer resources and configuration was 16 vs 2.5 days, respectively. Similar patterns of CPU timing are observed for water (Fig. S5). In addition, the advantage of using polarization- instead of correlation-consistent basis sets becomes more important for larger molecules.

Fig. 3.

Fig. 3

CPU time (min) dependence on the type and size of basis set for formaldehyde VPT2 calculation with pc-n, cc-pVXZ and aug-cc-pVXZ basis sets

In the next step we tested the performance of several methods (RHF, MP2, B3LYP, BLYP, B3PW91 and PBE) in predicting anharmonic frequencies of water and formaldehyde at different Jensen’s basis set sizes (pc-2 and pc-4) and compared the results with those from two often used Pople’s basis sets (6-31G and 6-311++G**). The results obtained for water harmonic and anharmonic frequency deviations from experiment are shown in Table 3; similar data for formaldehyde are shown in Table 4. Contrary to formaldehyde anharmonic results obtained from B3LYP calculations discussed earlier, there was no influence of grid size on water and formaldehyde anharmonic deviations at BLYP, B3PW91 and PBE level. Therefore, only results for large grids and tight SCF convergence criteria are presented in Tables 3 and 4. However, for the sake of comparison, all results are presented in Tables S4S7.

Table 3.

Deviations from experimental values of water harmonic (Δharm), anharmonic (Δanh) and scaled harmonic (Δscal) frequencies (cm−1) calculated with different methods and Pople or Jensen’s basis sets

RHF/6-31G RHF/6-311++G** MP2/6-31G MP2/6-311++G** B3LYP/6-31G B3LYP/6-311++G**
Mode exp.a Δharm Δanh Δscal Δharm Δanh Δscal Δharm Δanh Δscal Δharm Δanh Δscal Δharm Δanh Δscal Δharm Δanh Δscal
νas(OH) 3756 389 200 −45 489 316 95 77 −131 −137 247 62 74 27 −174 −120 166 −19 43
νs(OH) 3657 332 160 −86 486 324 102 0 −189 −205 228 50 60 −40 −217 −180 160 −13 40
δ(HOH) 1595 142 75 −40 131 81 −29 68 6 −25 34 −17 −37 24 −34 −39 8 −39 −43
RMS 306 154 61 405 265 82 59 133 143 195 47 59 31 162 127 133 26 42
BLYP/6-31G BLYP/6-311++G** B3PW91/6-31G B3PW91/6-311++G** PBE/6-31G PBE/6-311++G**
Mode exp. Δharm Δanh Δscal Δharm Δanh Δscal Δharm Δanh Δscal Δharm Δanh Δscal Δharm Δanh Δscal Δharm Δanh Δscal
νas(OH) 3756 −147 −349 −169 23 −169 21 74 −126 −90 202 18 65 102 −96 53 231 49 211
νs(OH) 3657 −212 −386 −233 17 −163 15 4 −173 −153 194 21 60 33 −144 −14 222 51 202
δ(HOH) 1595 −3 −57 −12 −24 −70 −25 27 −32 −43 10 −36 −46 34 −25 13 15 −31 7
RMS 149 302 166 21 142 20 45 125 106 162 26 57 65 101 33 185 44 169
RHF/pc-2 RHF/pc-4 MP2/pc-2 MP2/pc-4 B3LYP/pc-2 B3LYP/pc-4
Mode exp. Δharm Δanh Δscal Δharm Δanh Δscal Δharm Δanh Δscal Δharm Δanh Δscal Δharm Δanh Δscal Δharm Δanh Δscal
νas(OH) 3756 483 305 108 475 302 99 235 45 65 215 31 66 159 −28 38 153 −30 55
νs(OH) 3657 481 316 114 473 313 106 210 33 46 189 15 44 155 −19 37 150 −20 55
δ(HOH) 1595 149 92 −6 153 97 −2 43 −12 −26 42 −13 −19 30 −23 −20 34 −18 −6
RMS 403 259 91 397 258 84 184 33 48 167 21 47 129 23 33 125 23 45
BLYP/pc-2 BLYP/pc-4 B3PW91/pc-2 B3PW91/pc-4 PBE/pc-2 PBE/pc-4
Mode exp. Δharm Δanh Δscal Δharm Δanh Δscal Δharm Δanh Δscal Δharm Δanh Δscal Δharm Δanh Δscal Δharm Δanh Δscal
νas(OH) 3756 16 −178 13 11 −179 9 195 9 57 187 6 50 221 37 201 214 34 194
νs(OH) 3657 11 −168 9 9 −167 7 188 16 54 182 13 48 214 44 194 209 41 188
δ(HOH) 1595 −1 −53 −2 3 −49 2 32 −21 −25 36 −16 −20 35 −17 27 40 −12 32
RMS 11 144 9 8 144 7 157 16 48 152 12 42 179 35 162 174 32 157

aFrom [40]

Table 4.

Deviations of formaldehyde harmonic (Δharm), anharmonic (Δanh) and scaled harmonic (Δscal) frequencies (cm−1) calculated using different methods and Pople or Jensen’s basis sets from the experimental values

RHF/6-31G RHF/6-311++G** MP2/6-31G MP2/6-311++G** B3LYP/6-31G B3LYP/6-311++G**
Mode exp.a Δharm Δanh Δscal Δharm Δanh Δscal Δharm Δanh Δscal Δharm Δanh Δscal Δharm Δanh Δscal Δharm Δanh Δscal
νas(CH2) 2843 457 262 111 326 210 32 263 144 89 204 21 73 191 −15 74 98 −29 7
νs(CH2) 2782 426 272 90 315 182 28 238 71 69 193 45 65 181 18 66 101 −50 11
ν(CO) 1745 165 135 −35 251 226 66 −65 −107 −159 17 −14 −59 3 −27 −64 69 44 13
σ(CH2) 1500 173 141 −2 150 120 −3 15 −10 −69 59 26 −8 60 28 0 31 −1 −17
ρ(CH2) 1250 124 103 −20 113 96 −13 34 11 −38 29 8 −26 24 2 −25 10 −9 −29
τ(CH2) 1167 162 143 22 169 150 45 30 8 −37 39 19 −13 36 15 −11 35 14 −3
RMS 285 188 61 235 171 37 148 79 87 119 25 48 112 20 50 67 30 16
BLYP/6-31G BLYP/6-311++G** B3PW91/6-31G B3PW91/6-311++G** PBE/6-31G PBE/6-311++G**
Mode exp. Δharm Δanh Δscal Δharm Δanh Δscal Δharm Δanh Δscal Δharm Δanh Δscal Δharm Δanh Δscal Δharm Δanh Δscal
νas(CH2) 2843 63 −149 46 −13 −260 −15 204 0 73 107 −20 5 228 27 190 128 −1 113
νs(CH2) 2782 68 −101 51 3 −157 1 192 30 64 109 −40 9 212 52 175 127 −21 112
ν(CO) 1745 −61 −93 −71 −10 −37 −11 13 −17 −62 89 64 25 29 −1 6 107 82 97
σ(CH2) 1500 13 −19 4 −11 −43 −12 63 30 −4 29 −2 −24 69 37 50 32 1 24
ρ(CH2) 1250 −14 −37 −21 −28 −48 −29 26 4 −29 9 −9 −35 31 9 15 13 −6 6
τ(CH2) 1167 −8 −29 −15 −10 −32 −11 37 18 −14 36 15 −6 45 25 30 41 20 35
RMS 46 85 42 15 128 15 119 20 49 75 33 21 133 30 108 88 36 78
RHF/pc-2 RHF/pc-4 MP2/pc-2 MP2/pc-3 B3LYP/pc-2 B3LYP/pc-4
Mode exp. Δharm Δanh Δscal Δharm Δanh Δscal Δharm Δanh Δscal Δharm Δanh Δscal Δharm Δanh Δscal Δharm Δanh Δscal
νas(CH2) 2843 321 208 41 312 203 31 233 42 102 216 31 101 101 −26 10 95 −29 22
νs(CH2) 2782 312 179 38 304 173 30 216 66 89 199 51 87 103 −49 14 99 −51 27
ν(CO) 1745 249 224 72 249 224 72 21 −11 −54 20 −12 −47 71 45 15 71 44 26
σ(CH2) 1500 154 122 8 151 119 4 57 24 −9 47 14 −12 34 1 −13 32 −1 −6
ρ(CH2) 1250 123 102 1 122 102 0 31 9 −23 26 4 −22 18 −4 −21 18 −4 −14
τ(CH2) 1167 174 156 55 170 153 51 45 25 −6 35 17 −11 37 18 0 33 15 3
RMS 235 171 44 230 168 40 134 35 60 123 26 59 69 30 14 66 31 19
BLYP/pc-2 BLYP/pc-4 B3PW91/pc-2 B3PW91/pc-4 PBE/pc-2 PBE/pc-4
Mode exp. Δharm Δanh Δscal Δharm Δanh Δscal Δharm Δanh Δscal Δharm Δanh Δscal Δharm Δanh Δscal Δharm Δanh Δscal
νas(CH2) 2843 −10 −257 −12 −14 −258 −15 106 −22 3 100 −26 −2 125 −5 109 119 −9 103
νs(CH2) 2782 6 −154 4 3 −154 2 108 −42 7 103 −44 3 123 −25 108 119 −27 104
ν(CO) 1745 −6 −33 −7 −8 −34 −9 89 63 25 89 61 25 105 80 95 105 79 95
σ(CH2) 1500 −7 −42 −8 −9 −43 −10 30 −4 −24 28 −7 −26 32 0 24 30 −3 22
ρ(CH2) 1250 −20 −43 −21 −20 −43 −21 15 -7 −29 15 −7 −29 18 −4 11 17 −4 11
τ(CH2) 1167 −9 −29 −9 −13 −32 −14 35 16 −7 31 12 −11 41 22 35 37 19 30
RMS 11 126 12 12 127 13 74 33 19 71 33 19 86 35 76 83 35 73

aFrom [41]

In the case of RHF calculations, both harmonic and anharmonic (although these are considerably better) frequencies obtained with both Pople and Jensen’s basis sets significantly overestimate experimental water and formaldehyde frequencies. The MP2 anharmonic values obtained with the 6-31G basis set for water and formaldehyde are not very accurate, but increasing the size of the basis set significantly improves the results. On the other hand, MP2 calculations are extremely expensive and feasible for very small molecules only. Water harmonic values obtained at the BLYP/6-31G level underestimate experimental frequencies, and anharmonic calculation using the PT2 method leads to their severe underestimation. Accidental error cancellation leads to very accurate BLYP calculated water harmonic frequencies but the corresponding anharmonic values are too low (Table 3). In the case of formaldehyde, harmonic frequencies calculated at BLYP level using larger basis sets are fairly accurate, while the corresponding anharmonic values are too small. Hence, paradoxically, formaldehyde anharmonic vibrations calculated at the BLYP level with larger basis sets exhibit worse RMS values. In the case of B3PW91 and PBE density functionals, similar improvements to those observed for B3LYP are obtained in case of formaldehyde anharmonic frequencies for larger basis sets (Tables 3, 4). However, it should be noted that, contrary to B3LYP, very good anharmonic results are obtained for formaldehyde by using the default grid size with B3PW91 and PBE density functionals (see Tables S6, S7). This makes B3LYP a more expensive DFT method for anharmonic calculations of some molecules. Therefore, to gain a more general insight, similar studies on the accuracy and reliability of the VPT2 method in predicting fundamental vibrations for a larger set of model molecules are planned.

Conclusions

In this paper we show, for the first time, the convergence of harmonic and anharmonic (calculated using VPT2 method) water and formaldehyde frequencies toward the B3LYP/pc-n and B3LYP/(aug)-cc-pVXZ CBS.

  1. The convergence of harmonic and anharmonic frequencies with respect to basis set size shows that pc-n basis sets consistently perform better than Pople basis sets. Both correlation-consistent and polarization-consistent basis sets enable essentially the same CBS values of harmonic and anharmonic frequencies to be obtained. However, the CPU time for calculations using cc-pVXZ basis sets is significantly longer than with the corresponding pc-n sets. The deviations in CBS values for harmonic frequencies are significantly larger than the corresponding anharmonic numbers (RMS of 119 vs 24 cm−1 in the case of water frequencies calculated using B3LYP/pc-n, and 62 vs 32 cm−1 in the case of formaldehyde frequencies). However, RMS deviations after simple scaling of harmonic frequencies are in most cases smaller and easier to obtain (39 and 16 cm−1, for water and formaldehyde, respectively). On the other hand, there are as yet no available scaling factors for Jensen’s basis set. Thus, arbitrary scaling factors were used for harmonic frequencies calculated with polarization-consistent basis sets.

  2. There is no point in using the VPT2 method in conjunction with the RHF and BLYP methods (the former values are far too high, and for the second method the anharmonic frequencies are too low).

  3. Optimization criteria and density grid size have a negligible effect on the harmonic frequencies of water and formaldehyde, but could significantly influence the corresponding anharmonic vibrations. For example, in more demanding calculations (OPT=very tight, SCF=tight and INT(GRID = 150590), the B3LYP-calculated formaldehyde anharmonic frequencies with large basis sets are significantly closer to experimental values.

The anharmonic frequencies depend on many points on the potential energy surface (PES) away from the equilibrium, and the method of calculation applied should produce very smooth PES (with constant errors). This could explain the high sensitivity of formaldehyde anharmonic frequencies to grid size, in contrast to harmonic vibrations. With the default grid size (sparse points), energy variations are not smooth and could lead to significant changes in anharmonic frequencies. On the basis of the results obtained here, we would stress the need for further study in this field.

Electronic supplementary material

Below is the link to the electronic supplementary material.

894_2010_913_MOESM1_ESM.doc (482KB, doc)

Additional figures (convergence of water and formaldehyde harmonic and anharmonic frequencies as well as CPU timing for water) and tables (applied scaling factors and the influence of grid size and SCF convergence criteria on harmonic and anharmonic frequency deviations) are gathered in supporting material. This material is made available free of charge via the Internet at http://pubs.acs.org. (DOC 482 kb)

Figure S1 (162KB, doc)

Convergence of water harmonic and anharmonic frequencies calculated at B3LYP/cc-pVXZ level. The CBS (Q-6) values estimated using Eq. 1. (DOC 162 kb)

Figure S2 (162KB, doc)

Convergence of water harmonic and anharmonic frequencies calculated at B3LYP/aug-cc-pVXZ level. The CBS (Q-6) estimated using Eq. 1. (DOC 162 kb)

Figure S3 (323KB, doc)

Convergence of formaldehyde harmonic and anharmonic frequencies calculated at B3LYP/cc-pVXZ level. The CBS (Q-6) estimated using Eq. 1. (DOC 323 kb)

Figure S4 (427KB, doc)

Convergence of formaldehyde harmonic and anharmonic frequencies calculated at B3LYP/aug-cc-pVXZ level. The CBS (Q,5) estimated using Eq. 1. (DOC 427 kb)

Figure S5 (88.5KB, doc)

CPU time (min) dependence on the type and size of basis set for water VPT2 calculation with pc-n, cc-pVXZ and aug-cc-pVXZ basis sets. (DOC 88 kb)

Acknowledgments

Aneta Buczek is a recipient of a PhD fellowship from a project funded by the European Social Fund. Calculations were carried out in Wroclaw Centre for Networking and Supercomputing (http://www.wcss.wroc.pl), and in the Academic Computer Centre CYFRONET, AGH, Kraków, grant MEiN/SGI3700/UOpolski/063/2006. T. K. was supported by grant 10/WCH/2010-S.

Open Access

This article is distributed under the terms of the Creative Commons Attribution Noncommercial License which permits any noncommercial use, distribution, and reproduction in any medium, provided the original author(s) and source are credited.

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Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Supplementary Materials

894_2010_913_MOESM1_ESM.doc (482KB, doc)

Additional figures (convergence of water and formaldehyde harmonic and anharmonic frequencies as well as CPU timing for water) and tables (applied scaling factors and the influence of grid size and SCF convergence criteria on harmonic and anharmonic frequency deviations) are gathered in supporting material. This material is made available free of charge via the Internet at http://pubs.acs.org. (DOC 482 kb)

Figure S1 (162KB, doc)

Convergence of water harmonic and anharmonic frequencies calculated at B3LYP/cc-pVXZ level. The CBS (Q-6) values estimated using Eq. 1. (DOC 162 kb)

Figure S2 (162KB, doc)

Convergence of water harmonic and anharmonic frequencies calculated at B3LYP/aug-cc-pVXZ level. The CBS (Q-6) estimated using Eq. 1. (DOC 162 kb)

Figure S3 (323KB, doc)

Convergence of formaldehyde harmonic and anharmonic frequencies calculated at B3LYP/cc-pVXZ level. The CBS (Q-6) estimated using Eq. 1. (DOC 323 kb)

Figure S4 (427KB, doc)

Convergence of formaldehyde harmonic and anharmonic frequencies calculated at B3LYP/aug-cc-pVXZ level. The CBS (Q,5) estimated using Eq. 1. (DOC 427 kb)

Figure S5 (88.5KB, doc)

CPU time (min) dependence on the type and size of basis set for water VPT2 calculation with pc-n, cc-pVXZ and aug-cc-pVXZ basis sets. (DOC 88 kb)


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