37.3 DFTSAPT
It is of crucial importance to account for the intramolecular
correlation effects of the individual SAPT terms since
HartreeFock theory often yields poor first and secondorder
electrostatic properties. While this can be done
using manybody perturbation theory [1] (in a double perturbation theory
ansatz) a more efficient way is to use static and
timedependent DFT theory. This variant of SAPT, termed as
DFTSAPT [26], has in contrast to HartreeFockSAPT the appealing
feature that the polarisation terms (
,
,
) are
potentially exact, i.e. they come out exactly if the
exact exchangecorrelation (xc) potential and the exact
(frequencydependent) xc response kernel of the monomers
were known. On the other hand, this does not hold for the
exchange terms since KohnSham theory can at best give
a good approximation to the exact density matrix of a
manybody system. It has been shown [6] that this is indeed the
the case and therefore DFTSAPT has the potential to
produce highly accurate interaction energies comparable
to highlevel supermolecular manybody perturbation or coupled cluster
theory. However, in order to achieve this accuracy, it
is of crucial importance to correct the wrong asymptotic
behaviour of the xc potential in current DFT functionals
[35]. This can be done by using e.g.:
{ks,lda; asymp,<shift>}
which activates the gradientregulated asymptotic correction approach of
Grüning et al. (J. Chem. Phys. 114, 652 (2001)) for the
respective monomer calculation. The user has to supply a shift
parameter (
) for the bulk potential which should
approximate the difference between the HOMO energy
(
) obtained from the respective standard KohnSham
calculation and the (negative) ionisation potential of the
monomer ():

(57) 
This method accounts for the derivative discontinuity of the exact
xcpotential and that is missing in approximate ones.
Note that this needs to be done only once for each system. (See also section
37.7.2 for an explicit example).
Concerning the more technical parameters in the DFT monomer calculations it is
recommended to use lower convergence thresholds and larger intergration grids
compared to standard KohnSham calculations.
molpro@molpro.net 20190122