Computational Chemistry

functional or semiempirical), since frequencies must be calculated using the same
method used for the geometry optimization (Section2.5).

3.6.2 Weaknesses

The possible pitfalls in using MM are discussed by Lipkowitz [ 33 ]. The weaknesses
stem from the fact that it ignores electrons. The philosophy behind MM is to think
of a molecule as a collection of atoms subject to forces and to use any practical
mathematical treatment of these forces to express the energy in terms of the
geometric parameters. By parameterization MM can “calculate” electronic proper-
ties; for example, using bond dipoles it can find a dipole moment for a molecule,
and using values that have been calculated for various atom types by quantum
mechanics it can assign charges to atoms. However, such results are obtained purely
by analogy, and their reliability can be negated by unexpected electronic factors to

0

4000

0

04000 3500

MMFF

3000 2500 2000

FREQ_VAL

FREQ_VAL

1500 1000 500

4000 3500 3000 2500 2000 1500 1000 500

73

30

60

90

MP2 / 6-31G*

1.5e+002

2.2e+002

3000

1739

(^13661217)
1636
2000 1000
20
Acetone
Experimental
40
60
80
IR_INTENS
IR_INTENS
1792
1787
1231
569540
1448
1286
1534
1549 537
Fig. 3.14 Experimental (gas phase), and MM (MMFF) and ab initio (MP2(fc)/6–31G*) calcu-
lated IR spectra of acetone
74 3 Molecular Mechanics