Computational Chemistry

6.3.4.1 Dipole Moments

Hehre’s extensive survey of practical computational methods reports the results of
ab initio and DFT single point dipole moment (m) calculations on AM1 geometries
[ 106 ]. There does not appear to be much advantage to calculating HF/6-31G
dipole moments on HF/6-31G geometries (HF/6-31G//HF/6-31G calculations)
rather than on the much more quickly- obtained AM1 geometries (HF/6-31G//
AM1 calculations). Indeed, even the relatively time-consuming MP2/6-31G//
MP2/6-31G calculations seem to offer little advantage over fast HF/6-31G//AM1
calculations as far as dipole moments are concerned (Tables 2.19 and 2.21 in
ref. [ 106 ]). This is consistent with our finding that AM1 geometries are usually
quite good (Section6.3.1). Table 6.4compares calculated and experimental

80

60

40

20

0

0

57

1.1e+002

1.7e+002

4000

4000

3000

3500 3000

2000

2500 2000

1000

1500 1000 500

3050

Dichloromethane

Experimental

AM1

1218

876

774

757

(^12761261)
0
43
87
1.3e+002
4000 3500 3000 2500 2000 1500 1000 500
MP2 / 6-31G
1378
817
FREQ_VAL
Fig. 6.7 Experimental (gas phase), AM1 and ab initio (MP2(fc)/6–31G) calculated IR spectra of
dichloromethane
6.3 Applications of Semiempirical Methods 427