25.2.24 Keywords for Module ccsdf12

The ccsdf12 program uses a subset of the data groups of the ricc2 program. For F12 calculations it uses in addition the data groups $rir12, $lcg, $cabs, and $jkbas. See the respective sections for further details.

The ccsdf12 program recognizes in the $ricc2 data group the following options to specify wavefunction methods:

$ricc2

ccsd
bccd rohf
mp3
mp4
ccsd(t)
bccd(t) rohf
shift 0.0
core  can/res/bru
no_sc

The options mp3, mp4, and ccsd request, respectively, MP3, MP4 and CCSD calculations. The option ccsd(t) request a CCSD calculation with the perturbative triples correction, CCSD(T), and as a side result also the CCSD[T] energy will be printed. The option bccd requests a BCCD calculation, which is UBCCD for open-shell systems by default. The sub-option rohf for BCCD or BCCD(T) requests a ROHF-BCCD calculation, where the alpha and beta Brueckner orbitals are restricted to be the same for the doubly occupied orbitals of an ROHF open shell initial reference state.

shift

sets a level shift \((>0)\) for the amplitude update in the CC iterations, useful to improve convergence for CCSD or BCCD calculations on e.g. transition metal complexes.

core

specifies the nature of the core orbitals in a frozen core calculation as either the (semi-)canonicalised reference mos (can) or the unmodified reference mos (res) or Brueckner core orbitals (bru) that are relaxed during a BCCD calculation.

no_sc

suppresses the default semi-canonicalisation of a ROHF reference prior to a CCSD or BCCD calculation, such that the output amplitudes and pair energies correspond to the restricted orbitals. Note that a semi-canonicalisation step is always performed immediately before the evaluation of the (T) energy.

In the data group $rir12 it recognizes ccsdapprox as additional option:

ccsdapprox label

defines the approximation to CCSD-F12 which will be used if the MP2-F12 calculation is followed by a CCSD or CCSD(T) calculation. The available approximation and corresponding labels are

CCSD(F12)	ccsd(f12)
CCSD(F12*)	ccsd(f12*)
CCSD[F12]	ccsd[f12]
CCSD-F12b	ccsd-f12b
CCSD(2*)\(_{\overline{\mathrm{F12}}}\)	ccsd(2)_/f12
CCSD(2)\(_{\overline{\mathrm{F12}}}\)	ccsd(2)_/f12
CCSD\(_{\mathrm{(F12*)}}\)	ccsd_(f12*)

It is recommended that these approximations are only used in combination with ansatz 2 and the SP approach (i.e. geminal coefficients fixed by the cusp conditions). For CCSD-F12b calculations also the CCSD-F12a energies are calculated as a byproduct. By default a CCSD(F12*) calculation is carried out, because it gives the best cost/accuracy ration among the approximations. CCSD\(_{\mathrm{(F12*)}}\) is the perturbative variant of CCSD(F12*) and should be used when performing a Brueckner calculation.