Computational Chemistry

[ 88 , 107 ] dipole moments for ten molecules, using these methods: AM1 (using the
AM1 method to calculatemfor the AM1 geometry, AM1//AM1), HF/6-31//AM1,
PM3 (PM3//PM3), HF/6-31G//PM3, and MP2/6-31G (MP2/6-31G//MP2/6-
31G). For this set of molecules, the smallest deviation from experiment, as judged
by the arithmetic mean of the absolute deviations from the experimental values, is
shown by the AM1 calculation (0.21 D), and the largest deviation is shown by the
“highest” method, MP2/6-31G (0.34 D). The other three methods give essentially
the same errors (0.27–0.29 D). It is of course possible that AM1 gives the best
results (for this set on molecules, at least) because errors in geometry and errors in
the calculation of the electron distribution cancel. A study of 196 C, H, N, O, F, Cl,
Br, I molecules gave these mean absolute errors: AM1, 0.35 D; PM3, 0.40 D;
SAM1, 0.32 D [ 76 ]. Another study with 125 H, C, N, O, F, Al, Si, P, S, Cl, Br, I

80

60

40

20

IR_INTENS

0

37

73

1.1e+002

4000

4000

3000

3500 3000

2000

2500 2000

1000

1500 1000 500

Methanol

Experimental

(^37003673)
2981
2826
1016
1056 1033
1522
3501
(^37953223)
3144
3076
MP2 / 6-31G
1418
1083
AM1
1362
1132
2865, 2844
FREQ_VAL
FREQ_VAL
IR_INTENS
0
47
93
1.4e+002
4000 3500 3000 2500 2000 1500 1000 500
Fig. 6.8 Experimental (gas phase), AM1 and ab initio (MP2(fc)/6–31G) calculated IR spectra of
methanol
428 6 Semiempirical Calculations