35.4 High order terms

It has been found that third and higher-order terms become quite important if one or both monomers are polar. As no higher than second-order terms are currently implemented in SAPT, one may use a non-correlated estimation of those terms by using supermolecular Hartree-Fock (see e.g. [7]). This can be done by adapting the following template:

!dimer
hf
edm=energy

!monomer A
dummy,<monomer2>
{hf; save,$ca}
ema=energy
sapt;monomerA

!monomer B
dummy,<monomer1>
{hf; start,atdens; save,$cb}
emb=energy
sapt;monomerB

!interaction contributions
sapt,sapt_level=2;intermol,ca=$ca,cb=$cb

esup=(edm-ema-emb)*1000. mH
dHF=esup-e1pol-e1ex-e2ind-e2exind

which stores the resulting $\delta$(HF) term in dHF.