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CCSD, CCSD(F12*) and CCSD(T) calculations

CCSD, CCSD(F12*) and CCSD(T)

The ricc2 module includes also an implementation of the full coupled-cluster singles-and-doubles method CCSD and its explicitly-correlated CCSD(F12) and CCSD(F12*) variants. CCSD and the F12 variants can be combined with a perturbative correction for connected triple excitations, CCSD(T).% latex2html id marker 60972
\setcounter{footnote}{1}\fnsymbol{footnote}As perturbative approximations beyond MP2 also the approximations MP3, MP3(F12), MP4, and MP4(F12*) are available. Presently the implementation of the F12 variants and of connected triple excitations is restricted to ground state energies and the CCSD implementation to ground-state and excitation energies. Closed-shell (RHF), unrestricted (UHF) or single determinant restricted (ROHF) open-shell reference wavefunctions can be used for CCSD and CCSD(T), but no gradients or properties are (yet) available for these wavefunction models. The MP3 and MP4 approximations can currently not be combined with ROHF reference wavefunctions.

Further limitations:

no MPI parallelization
, calculations at these levels can presently only carried out on a single compute node, only the OpenMP (see Sec. 3.2.1) parallelization is available for calculations beyond CC2.
use of symmetry restricted
to D2h and its subgroups for the conventional implementation; no symmetry can be used for the F12 methods

Please note that calculations with MP3, MP4, CCSD and methods beyond CCSD require considerably more disc space and core memory than MP2 or CC2 calculations. (See section below for more details and recommendations.)



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