Table 2. HOMO–1, HOMO, and LUMO Energy Levels (in eV) for the DNA/RNA Nucleobases Calculated at Different Levels of Theory^a.

	LDA-KSb	B3LYP/cc-pVTZ	LC-BLYP/cc-pVTZ	GWb	CASPT2b,c	experimentb
Guanine
HOMO–1	6.34	7.11	9.29	9.82	9.56	9.9
HOMO	5.69	5.75	7.78	7.81	8.09	8.0–8.3
LUMO	1.8	0.30	–1.69	–1.58	–1.14
fundamental gap	3.89	5.45	9.47	9.39	9.23
Adenine
HOMO–1	6.28	6.98	9.21	9.47	9.05	9.45
HOMO	6.02	6.11	8.21	8.22	8.37	8.3–8.5, 8.47
LUMO	2.22	0.81	–1.13	–1.14	–0.91	–0.56 to −0.45
fundamental gap	3.80	5.30	9.33	9.36	9.28
Cytosine
HOMO–1	6.172	6.94	9.37	9.52	9.42	9.45, 9.55
HOMO	6.167	6.45	8.73	8.73	8.73	8.8–9.0, 8.89
LUMO	2.57	1.20	–0.93	–0.91	–0.69	–0.55 to −0.32
fundamental gap	3.60	5.25	9.66	9.64	9.42
Thymine
HOMO–1	6.68	7.50	9.71	10.41	9.81	9.95–10.05, 10.14
HOMO	6.54	6.81	8.90	9.05	9.07	9.0–9.2, 9.19
LUMO	2.83	1.47	–0.59	–0.67	–0.6	–0.53 to −0.29
fundamental gap	3.71	5.33	9.49	9.72	9.67
Uracil
HOMO–1	6.88	7.53	9.99	10.54	9.83	10.02–10.13
HOMO	6.72	7.15	9.45	9.47	9.42	9.4–9.6
LUMO	3.01	1.63	–0.53	–0.64	–0.61	–0.30 to −0.22
fundamental gap	3.71	5.52	9.98	10.11	10.03
MAE HOMO–1	3.06 (3.48)	2.32 (2.74)	0.02 (0.44)	0.42
MAE HOMO	2.51 (2.43)	2.28 (2.20)	0.12 (0.04)	0.08
MAE LUMO	3.28 (3.47)	1.87 (2.07)	0.18 (0.02)	0.20
MAE fundamental gap	5.78 (5.90)	4.16 (4.27)	0.06 (0.06)	0.12

^aNote: the negatives of the orbital energies are reported. The fundamental gap is determined by the difference between the HOMO and LUMO orbital energies. The MAE (mean absolute error) values are with respect to the CASPT2 values; the values in parentheses are with respect to the GW values.

^bValues were obtained from ref (1).

^cThe reported CASPT2 HOMO/LUMO energies are actually vertical ionization energies and electron affinities, respectively (see text).