Q-Chem’s implementation of SAPT0 was designed from the start as a correction for XPol calculations,
affording the “XSAPT” method that is described in Section 12.14. As such, even a traditional SAPT0
calculation is requested by setting JOBTYPE = XSAPT. However, whereas XSAPT calculations
are based on XPol wave functions for the monomers, which can
capture many-body polarization effects in systems composed
of more than two monomers (see Section 12.14), traditional SAPT calculations are based instead on
gas-phase monomer wave functions. This can be realized by turning off the XPol charge embedding, *i.e.*, by
setting embed = none in the *$xpol* section that was introduced in Section 12.12.

Energy components are printed separately at the end of a SAPT job. If
EXCHANGE = HF, then an XSAPT calculation with XPol embedding turned off corresponds
to a SAPT0 calculation. Alternatively, if a density functional level of theory is requested in the *$rem* section,
then JOBTYPE = XSAPT will perform a SAPT(KS) calculation, *i.e.*, one that is based on
a Kohn-Sham description of the monomers.

Note:
(1) Meta-GGAs are not yet available for SAPT(KS) calculations when the projected
(pseudocanonicalized) basis set is used. SAPT(KS) calculations can be performed with
meta-GGAs using the monomer or dimer basis sets.
(2) Both closed- and open-shell (unrestricted) SAPT(KS) calculations are available.
(3) Frozen orbitals are *not* available for use with SAPT(KS).

The remaining job control options for SAPT calculations are specified using various keywords contained in
a *$sapt* input section, as described below.
Researchers who use Q-Chem’s SAPT code are asked to cite Refs. Jacobson:2011, Herbert:2012.

Algorithm

Specifies which SAPT algorithm will be used

INPUT SECTION: *$sapt*

TYPE:

STRING

DEFAULT:

MO

OPTIONS:

MO
Traditional molecular orbital-based algorithm
RI-MO
MO-based algorithm with resolution-of-identity approximation
AO
Atomic orbital-based algorithm.

RECOMMENDATION:

The standard MO-based algorithm corresponds to an MP2-like implementation of
Eq. (12.57), where the RI-MO algorithm corresponds to an RIMP2-like implementation.
The RI implementation is generally much faster and introduces negligible errors (as compared to the standard implementation),
provided that the auxiliary basis set is matched to the primary basis set. (The former must be specified using
(AUX_BASIS in the *$rem* section.) The AO-based algorithm
does not implement Eq. (12.57) and is intended only for use with XSAPT(KS)+*ai*D
calculations; see Section 12.14.2.

Exchange

Specifies how the first-order exchange interaction will be evaluated.

INPUT SECTION: *$sapt*

TYPE:

STRING

DEFAULT:

S_Squared

OPTIONS:

S_Squared
Use the single-exchange (“${S}^{2}$") approximation.
S_Inverse
Compute the exact first order exchange.

RECOMMENDATION:

The single-exchange approximation is expected to be adequate except possibly at very short
intermolecular distances, and is somewhat faster to compute.

Basis

Controls which basis is used to evaluate the SAPT corrections

INPUT SECTION: *$sapt*

TYPE:

STRING

DEFAULT:

monomer

OPTIONS:

monomer
Use the monomer-centered basis set (MCBS).
dimer
Use the dimer-centered basis set (DCBS).
projected
Use the projected (pseudocanonicalized) basis set.

RECOMMENDATION:

The DCBS (in which the monomer wave functions are iterated to convergence using the
dimer basis set) is the preferred choice in traditional SAPT, although it is more costly than
the MCBS (which uses only the monomer basis set for the monomer wave functions).
The DCBS is ill-defined, and therefore unavailable, for use with XPol charge embedding.
The projected basis set is an efficient approximation to the DCBS for traditional SAPT
calculations,^{Jacobson:2011} and *is* available for use with XPol embedding.

CPHF

Requests that the second-order corrections ${E}_{\mathrm{ind}}^{(2)}$ and ${E}_{\mathrm{exch}\text{-}\mathrm{ind}}^{(2)}$
be replaced by their infinite-order “response” analogues, ${E}_{\mathrm{ind},\mathrm{resp}}^{(2)}$ and
${E}_{\mathrm{exch}\text{-}\mathrm{ind},\mathrm{resp}}^{(2)}$.

INPUT SECTION: *$sapt*

TYPE:

None

DEFAULT:

Not specified

OPTIONS:

Response quantities are calculated if the keyword is present.

RECOMMENDATION:

Computing the response corrections requires solving CPHF equations for each pair
of monomers, which is somewhat expensive but may improve the accuracy, especially when
the monomers are polar and induction contributions are large.

DSCF

Requests the $\delta {E}_{\mathrm{int}}^{\mathrm{HF}}$ correction

INPUT SECTION: *$sapt*

TYPE:

None

DEFAULT:

Not specified

OPTIONS:

The $\delta {E}_{\mathrm{int}}^{\mathrm{HF}}$ correction is computed if this keyword is present.

RECOMMENDATION:

Evaluating $\delta {E}_{\mathrm{int}}^{\mathrm{HF}}$ requires an SCF calculation on
the entire (super)system. In the context of SAPT0 calculations, this correction
essentially results in a
“Hartree-Fock plus dispersion” estimate of the interaction energy.

Print

Specifies the level of output for the XPol code.

INPUT SECTION: *$sapt*

TYPE:

INTEGER

DEFAULT:

1

OPTIONS:

$n$
Desired print level

RECOMMENDATION:

Higher values print additional information

$molecule 0 1 -- formamide 0 1 C -2.018649 0.052883 0.000000 O -1.452200 1.143634 0.000000 N -1.407770 -1.142484 0.000000 H -1.964596 -1.977036 0.000000 H -0.387244 -1.207782 0.000000 H -3.117061 -0.013701 0.000000 -- formamide 0 1 C 2.018649 -0.052883 0.000000 O 1.452200 -1.143634 0.000000 N 1.407770 1.142484 0.000000 H 1.964596 1.977036 0.000000 H 0.387244 1.207782 0.000000 H 3.117061 0.013701 0.000000 $end $rem JOBTYPE XSAPT BASIS AUG-CC-PVDZ AUX_BASIS RIMP2-AUG-CC-PVDZ METHOD HF $end $sapt algorithm ri-mo basis dimer $end