MOLPRO

33 SYMMETRY-ADAPTED INTERMOLECULAR PERTURBATION THEORY 249
sapt;monomerA
!monomer B
dummy,
{hf; start,atdens; save,$cb}
emb=energy
sapt;monomerB
!interaction contributions
sapt,sapt_level=2;intermol,ca=$ca,cb=$cb
esup=(edm-ema-emb)*1000. mH
dHF=esup-e1pol-e1ex-e2ind-e2exind
which stores the resulting δ(HF) term in dHF.
33.5 Density fitting
In order to be able to study interactions between extended monomers one can use density fitting
to approximate the integrals in SAPT [7]. For this one may use the input:
{sapt;intermol,ca=$ca,cb=$cb,fitlevel=3
dfit,basis_coul=jkfit,basis_exch=jkfit,basis_mp2=mp2fit,cfit_scf=3}
with in the basis section defined jkfit and mp2fit fitting basis sets (see section 15).
Currently only the ALDA xc-kernel is implemented for the case SAPT LEVEL=3 and SAPT FITLEVEL=3.
This means that a corresponding SAPT calculation would be uncompatible with hybrid-DFT
monomer orbitals/orbital energies. Therefore it is recommended to use nonhybrid functionals in
the case the dispersion/exchange-dispersion energy terms are requested in a DF-DFT-SAPT run.
Another possibility is to localise the xc-potential via, e.g., the OEP method (see also example in
section 33.7.3).
33.6 SAPT with ECP’s
If effective core potentials (ECP’s) are used in the monomer calculations, it is important to add
the δ(HF) term to the SAPT interaction energy (see K. Patkowski, K. Szalewicz, J. Chem. Phys.
127 (2007) 164103). For examples for the calculation of δ(HF) see sections 33.4 and 33.7.