TABLE I.

Equilibrium structures of 1,3-butadiene calculated by full VMC geometry optimizations are reported, together with other quantum chemistry calculations and experimental data. VMC₁ and VMC₂ stand for the calculations with the smaller and the larger basis sets, respectively, whereas (ECP) and (NCP) refer to the two pseudopotentials used to treat the core electrons of the carbon atoms, as described in Subsection II C.

	s-trans				gauche				s-cis
	BLA (Å)	RC1C2 (Å)	RC2C3 (Å)	θC1C2C3 (deg)	RC1C2 (Å)	RC2C3 (Å)	θC1C2C3 (deg)	ΔC1C2C3C4 (deg)	RC1C2 (Å)	RC2C3 (Å)	θc1c2C3 (deg)
VMC1 (ECP)	0.1257(4)	1.3326(2)	1.4583(4)	123.66(2)	1.3313(2)	1.4694(3)	124.65(1)	143.09(2)	1.3323(1)	1.4726(2)	126.79(1)
VMC2(ECP)	0.1252(2)	1.3328(1)	1.4580(2)	123.80(2)	1.3310(2)	1.4710(2)	125.26(1)	143.85(5)	1.3319(1)	1.4722(3)	126.57(1)
VMC1 (NCP)	0.1249(3)	1.3384(1)	1.4633(3)	123.65(2)					1.3389(2)	1.4767(5)	126.44(2)
VMC2(NCP)	0.1258(4)	1.3387(2)	1.4645(4)	123.82(3)					1.3387(2)	1.4766(4)	126.63(2)
CASSCFa	0.118	1.345	1.463	124.1
RASSCFa	0.117	1.351	1.468	123.9
CASSCFb	0.128	1.344	1.467	124.0					1.345	1.476	126.9
CASPT2b	0.106	1.348	1.454	123.6					1.351	1.468	126.7
CCSD(T)c	0.1160	1.3389	1.4549	123.5	1.3362	1.4682	124.4	144.5	1.3371	1.4696	126.3
MP2b	0.113	1.343	1.456	123.7					1.342	1.470	126.5
MP2d	0.1132	1.3401	1.4533	123.54
MP2e	0.1137	1.3425	1.4562	123.73	1.3415	1.4680	124.12	142.19
B3LYPb	0.117	1.339	1.456	124.3					1.339	1.470	127.3
B3LYPd	0.1188	1.3339	1.4527	124.36
B3LYPf	0.1183	1.3344	1.4527	124.35
B3LYPg	0.12	1.34	1.46	124.27	1.34	1.47	125.87	147.19
Semi-Exph	0.1163(14)	1.3376(10)	1.4539(10)	123.62(10)
SEDi	0.1233(14)	1.3439(5)	1.4672(13)	122.8(5)
EDj	0.122(3)	1.341(2)	1.463(3)	123.3(5)
EDk	0.118(2)	1.349(1)	1.467(2)	124.4(1)
MFTSl	0.130	1.337	1.467	123.5
EDm	0.146	1.337	1.483	122.4

^aCASSCF(4,8)/6-31G*+3p and RASSCF(22,9+5+12)[1,1]/6-31G*+3p are taken from Ref. 26.

^bCalculations with 6-31G(d) basis set from Ref. 24.

^cCCSD(T)(FC)/CBS calculations from Ref. 29, that include core/valence (CV) and scalar relativistic (SR) corrections. For the geometry of the s-trans conformer CCSDT(Q)(FC)/cc-pVDZ corrections are considered.

^dBoth calculations are made using the cc-pVTZ basis set, from Ref. 6.

^eCalculations with 6-31G* basis set from Ref. 23.

^fCalculations with auc-cc-pVTZ basis set from Ref. 28.

^gCalculations from Ref. 25.

^hSemi-experimental method from vibrational analysis described in Ref. 6.

ⁱSector electron diffraction from Ref. 4.

^jElectron diffraction from Ref. 5.

^kElectron diffraction from Ref. 75.

^lMicrowave Fourier transform spectrum from Ref. 76.

^mElectron diffraction from Ref. 3.