Computational Chemistry

to be bigger than experimental, and electron correlation, through DFT or MP2,
tends to lower the dipole moment, bringing it closer to the experimental value (e.g.
for thiophene, from 0.80 to 0.51 D for B3LYP; the MP2 value is 0.37 D and the
experimental dipole moment is 0.55 D [ 68 ]).
Table7.8compares with experiment dipole moments calculated by B3LYP/6-
31G, M06/6-31G, AM1 (as a check on this fast method), and MP2(fc)/6-31G*,
for ten molecules. The two DFT methods give the same mean unsigned error, 0.11
D, three times smaller than the error of 0.31 D from the slowest method, MP2 (at
least for this small selection of molecules), and the very fast AM1 moments lie in-
between, 0.22 D. None of these methods consistently gives values accurate to
within 0.1 D. Very accurate dipole moments (mean absolute deviation 0.06–0.07
D) can be obtained with gradient-corrected DFT and very large basis sets [ 74 ].

80

60

40

20

0

TR_TNTENS

0

57

1.1e+002

1.7e+002

4000

4000

3000

3500 3000

2000

2500 2000

1000

1500 1000 500

1276

1322

1378

817

733

1261

757

Dichloromethane

Experimental

B3LYP / 6-31G*

MP2 / 6-31G*

FREQ_VAL

TR_TNTENS

0

43

87

1.3e+002

4000 3500 3000 2500 2000 1500 1000 500

FREQ_VAL

Fig. 7.6 Experimental (gas phase), DFT (B3LYP/6-31G) and ab initio (MP2(fc)/6-31G)
calculated IR spectra of dichloromethane

488 7 Density Functional Calculations