Computations of Noncovalent Interactions - Reviews in Computational Chemistry - page 27

Under the SAPT2 approximation, the correlation energy is comprised of
the following individual contributions: 67

E ð12Þ E ð11Þ E ð12Þ

t E ð22Þ t E ð22Þ

E ð20Þ E ð20Þ E corr

int

¼

elst;resp þ exch þ exch þ ind þ exch - ind þ disp þ

½ 20

exch - disp

Again, cross terms involving two types of interactions appear—here, both
exchange-induction and exchange-dispersion terms. Based on a subjective ana-
lysis of SAPT2 results for interactions, we have decided somewhat arbi-

trarily to classify the exchange-induction and exchange-dispersion terms as
induction and dispersion, respectively. This yields the following scheme:

p - p

E ð10Þ E ð12Þ

elst

E ð electrostatic Þ¼

elst þ

½ 21

; resp

E ð10Þ E ð11Þ E ð12Þ

exch

E ð exchange Þ¼

exch þ exch þ

½ 22

E ð20Þ
ind

E ð20Þ

t E ð22Þ t E ð22Þ

exch - ind

E HF
int

E ð induction Þ¼

resp þ exch - ind;resp þ d

þ

ind þ

½ 23

; ; resp

E ð20Þ
disp

E ð20Þ

E ð dispersion Þ¼ þ ½ 24

exch - disp

Our group has obtained SAPT results using the programs of Szalewicz
and co-workers, which are freely available. 161 The Appendix gives an example
of the output from this program and how to extract the energetic components
in Eq. [21]-[24].
In our experience, the SAPT computations can be time demanding even
at second order (SAPT2), motivating the use of truncated basis sets. Fortu-
nately, as discussed above, a fortuitous cancellation of error exists when using
MP2 with the truncated aug-cc-pVDZ basis, aug-cc-pVDZ 0 , which removes
diffuse functions from hydrogen and diffuse d functions from all heavier
atoms. Because the SAPT2 results mimic MP2 for interaction energies, this
means that SAPT2/aug-cc-pVDZ 0 results should be fairly reliable.
As an example of the application of SAPT, consider the two C 2 v configura-
tions of the H 2 S-benzene complex 162,163 showninFigure6.Thehydrogens-down

H

H

S

S

H H

A B

Figure 6 Two symmetric configurations of the H 2 S-benzene complex.