comprehend the factors at work in a system, whether it be a galaxy, a supersonic
airliner, a thunderstorm, or a novel mathematical entity. We will briefly examine
below the role of computer graphics in computational chemistry, limiting ourselves
to molecular vibrations, van der Waals surfaces, charge distribution, and molecular
orbitals.
With due respect to the tremendous power ofvirtualmodels on a computer
screen or within virtual reality glasses [ 318 ], I feel it worthwhile to add, with a
small apology (for this is a book on computational chemistry) thatrealmolecular
models which you can hold and examine still have a place in chemistry. Professor
Roald Hoffmann has cautioned against slavish adherence to computer graphics and
commended traditional molecular models, saying there is no substitute to “running
one’s hands” over a molecular model and experiencing “the visual-tactile link [that
is] so important for establishing three-dimensionality in our minds....What I believe
is that the two generations of chemists who have seen molecules only on a screen
Table 5.18 The raw data for Table5.17: energies, ZPEs and HOMO values, for
calculating ionization energies
HF/6–31G* MP2(fc)/6–31G*
CH 3 OH #115.03542 #115.34514
0.05055 0.05086
#114.98487 #115.29528
CH 3 OH+cation geom. #114.68722 #114.95358
0.04723 0.04665
#114.63999 #114.90693
CH 3 OH+neutral geom. #114.6804 #114.94849
CH 3 OH, HOMO #0.44328 #0.44526
CH 3 SH #437.70032 #437.95267
0.04534 0.04621
#437.65498 #437.90646
CH 3 SH+cation geom. #437.39316 #437.62211
0.04468 0.04526
#437.34848 #437.57685
CH 3 SH+neutral geom. #437.39227 #437.62089
CH 3 SH, HOMO #0.35596 #0.35627
CH 3 COCH 3 #191.96224 #192.52391
0.08214 0.08309
#191.88010 #192.44082
CH 3 COCH: 3 þcation geom. #191.65994 #192.16837
0.08071 0.08128
#191.57923 #192.08709
CH 3 COCH: 3 þneutral geom. #191.65451 #192.16448
CH 3 COCH 3 , HOMO #0.40692 #0.41119
The numbers are hartrees, and represent (other than the HOMO energies): for the
neutrals and the cations at the cation geometry, uncorrected ab initio energy, ZPE,
and corrected ab initio energy. The ZPEs shown have been multiplied [ 80 ] by
0.9135 (HF) or 0.9670 (MP2(fc)). For the cations at the neutral geometry, ZPE
was not used and is not shown. Adiabatic IE¼E(cation)#E(neutral), both
corrected for ZPE. Vertical IE¼E(cation)#E(neutral), both without ZPE. Hartrees
were converted to eV in Table5.17by multiplying by 27.2116.5.5 Applications of the Ab initio Method 365