MOLPRO

31 EXPLICITLY CORRELATED METHODS 238
implies a canonical 3C calculation with diagonal ansatz 3C, using fixed coefficient and hynrid
approximations. The combination of the options fix=1 and canonical=1 implies a noniterative
calculation of the energy and is recommended. The above is equivalent to all of the
following:
ANSATZ=3C(FIXC,HY1)
ANSATZ=3C(D,FIXC,HY1)
ANSATZ=3C(D,HY1,FIX),canonical=1
Note that the HF convergence threshold should be rather strict to obtain accurate results (use
ACCU,14 in the HF).
Numerous further options are for specialist use only and not described here. See explicit.registry
for a full list.
31.8 CABS Singles correction
By default, the perturbative CABS singles correction as described in J. Chem. Phys. 127,
221106 (2007) and J. Chem. Phys. 128, 154103 (2008) is included in the reference energy of all
MP2-F12 and CCSD-F12 calculations (closed and open-shell, except for LMP2-F12/3*A(loc),
which is done with a different program). The corrected reference energy is stored in variable
ENERGR, so that ENERGY-ENERGR are the total correlation energies. For the setting of other
variables by the F12 programs see section 31.12.
The singles correction can be turned off by option SINGLES=0, e.g.
MP2-F12,SINGLES=0
The contribution of core orbitals to the singles energy is not included by default, but can be
turned on by option CORE SINGLES, e.g.
MP2-F12,CORE SINGLES=1
However, we do not recommended the use of core singles, because they depend sensitively on
the CABS basis construction and do not offer significant improvements in relative energies.
31.9 Pair specific geminal exponents
Different Slater exponents can be used for core-core, core-valence and valence-valence pairs as
described in H.-J. Werner, G. Knizia, and F. R. Manby (Mol. Phys., submitted). The exponents
are specified using the GEM BETA option, e.g., GEM BETA=[1.0, 1.7, 2.5] (see options,
section 31.6). The three values are used for vv, cv, cc pairs, respectively. In most cases, core
pairs can be defined by using the CORE directive: all orbitals that are not core and do not belong
to the default valence shell are then treated as core. If part of the default valence shell is to be
taken as core (e.g., the 3d shell in first-row transition metals), the core can be defined via the
PRCORE variable, e.g., prcore=[4,2,2,1,4,2,2,1] for Cu 2 . This variable must then be
defined before the F12 calculation that should use it. The following example shows calculation
for Br 2 , in which the 3d shell is treated as core.