32.5 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 11).

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.