MOLPRO

33 SYMMETRY-ADAPTED INTERMOLECULAR PERTURBATION THEORY 248
33.3 DFT-SAPT
It is of crucial importance to account for the intramolecular correlation effects of the individual
SAPT terms since Hartree-Fock theory often yields poor first- and second-order electrostatic
properties. While this can be done using many-body perturbation theory [1] (in a double perturbation
theory ansatz) a more efficient way is to use static and time-dependent DFT theory. This
variant of SAPT, termed as DFT-SAPT [2-6], has in contrast to Hartree-Fock-SAPT the appealing
feature that the polarisation terms (E (1)
pol , E(2) ind , E(2) disp
) are potentially exact, i.e. they come out
exactly if the exact exchange-correlation (xc) potential and the exact (frequency-dependent) xc
response kernel of the monomers were known. On the other hand, this does not hold for the
exchange terms since Kohn-Sham theory can at best give a good approximation to the exact
density matrix of a many-body system. It has been shown [6] that this is indeed the the case and
therefore DFT-SAPT has the potential to produce highly accurate interaction energies comparable
to high-level supermolecular many-body 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 [3-5]. This can be done by using e.g.:
{ks,lda; asymp,}
which activates the gradient-regulated 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 (∆ xc ) for the bulk potential which should approximate the difference between
the HOMO energy (ε HOMO ) obtained from the respective standard Kohn-Sham calculation and
the (negative) ionisation potential of the monomer (IP):
∆ xc = ε HOMO − (−IP) (63)
This method accounts for the derivative discontinuity of the exact xc-potential and that is missing
in approximate ones. Note that this needs to be done only once for each system. (See also
section 33.7.2 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 Kohn-
Sham calculations.
33.4 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,
{hf; save,$ca}
ema=energy