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35.10 CCSD(T)-F12

The CCSD-F12 and UCCSD-F12 programs first do
DF-MP2-F12/3C(FIX) (closed-shell) or DF-RMP2-F12/3C(FIX) (open-shell) calculations,
and then perform the CCSD-F12 (UCCSD-F12) without density fitting. By default,
the CCSD-F12A and CCSD-F12B energies are both computed. A specific method can be
requested by appending A or B to the -F12 suffix. Furthermore, instead of the 3C(FIX) ansatz,
different ansätze (e.g. 3C) can be used. In this case the amplitudes of the
explicitly correlated terms are determined in the MP2-F12 calculation and kept fixed
in the CCSD-F12.

It should be noted that these methods involve approximations and do not yield the
exact CCSD-F12 energies. Preliminary experience has shown that the CCSD-F12A method
slightly overestimates the correlation energies, while CCSD-F12B underestimates
them. For AVDZ or AVTZ basis sets, CCSD-F12A usually gives very good results,
but for larger basis sets it may overestimate the basis set limit and converge from
below to the limit. Thus, convergence may not be monotonic, and extrapolation
of the correlation energies should not be attempted. CCSD-F12B usually converges
monotonically from below to the limit and gives best results for AVQZ and larger
basis sets. Thus, we currently recommend CCSD-F12A
for AVDZ and AVTZ basis sets, and CCSD-F12B for larger basis sets (rarely needed).

The perturbative triples correction can be invoked by using CCSD(T)-F12 or
UCCSD(T)-F12. There is no direct F12 correction to the triples, and therefore
the basis set error of the triples is not affected by the F12 (small changes
of the triples energy arise from the fact that the doubles amplitudes are
affected by the F12 terms). In many cases, a simple and pragmatic improvement
of the triples energy can be obtained by scaling the triples energy contribution as

This can be done automatically by setting option `SCALE_TRIP=1`, i.e.
`CCSD(T)-F12,SCALE_TRIP=1`

molpro@molpro.net 2019-03-18