# 18 Explicitly correlated methods

Explicitly correlated calculations provide a dramatic improvement of the basis set convergence of MP2 and CCSD correlation energies. Such calculations can be performed using the commands of the form

command, options

where command can be one of the following:

MP2-F12
Closed-shell canonical MP2-F12. The F12-corrections is computed using density fitting, and then added to the MP2 correlation energy obtained without density fitting. By default, ansatz 3C(FIX) is used. Other ansaätze, as fully described in J. Chem. Phys. 126, 164102 (2007) can also be used (cf. section 18.3).
DF-MP2-F12
As MP2-F12, but the DF-MP2 correlation energy is used. This is less expensive than MP2-F12 since the standard two-electron integrals and the non-density fitted MP2 energy need not to be computed.
DF-RMP2-F12
Spin-restricted open-shell DF-RMP2-F12 as described in J. Chem. Phys. 128, 154103 (2008) By default, ansatz 3C(FIX) is used, but ansatz 3C(D) can also be used (cf. sections 18.3).
CCSD-F12
Closed-shell CCSD-F12 approximations as described in J. Chem. Phys. 127, 221106 (2007) and J. Chem. Phys. 130, 054104 (2009). By default, the fixed amplitude ansatz 3C(FIX) is used and the CCSD-F12a and CCSD-F12b energies are computed. Optionally, the command can be appended by A or B, and then only the corresponding energy is computed. For more details see section 18.8.
CCSD(T)-F12
Same as CCSD-F12, but perturbative triples are added.
UCCSD-F12
Open-shell unrestricted UCCSD-F12 approximations as described in J. Chem. Phys. 130, 054104 (2009). Restricted open-shell Hartree-Fock (RHF) orbitals are used. Optionally, the command can be appended by A or B, and then only the corresponding energy is computed.
UCCSD(T)-F12
Same as UCCSD-F12, but perturbative triples are added.
DF-LMP2-F12
Closed-shell DF-MP2-F12/3*A(D) with localized orbitals. Results for the fixed amplitude method DF-MP2-F12/3*A(FIX) are also printed In order to use local RI and DF approximations as described in J. Chem. Phys. 130, 054106 (2009) special options are necessary, see user's manual.
DF-LCCSD(T)-F12
Closed-shell DF-LCCSD(T)-F12/3*A(FIX). This includes DF-LMP2-F12.
Note that in the local methods only ansatz 3*A should be used (default), and F12 is not available for OSV methods.

Subsections
molpro@molpro.net 2018-12-18