11 CCSD, CCSD(F12*) and CCSD(T) calculations
Since release V7.0 the coupled-cluster singles-and-doubles method CCSD and its explicitly-correlated variants CCSD(F12) and CCSD(F12*) are implemented in the ccsdf12 program. CCSD and the F12 variants can be combined with a perturbative correction for connected triple excitations, CCSD(T).1 The Brueckner variants BCCD and BCCD(T) with and without explicit correlation are also available. 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. The CCSD implementation without F12 is limited 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 same is true for BCCD and BCCD(T). The MP3 and MP4 approximations can currently not be combined with ROHF reference wavefunctions.
Further limitations:
- no MPI parallelization:
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calculations at these levels can presently only carried out on a single compute node, only the OpenMP (see Sec. 3.4.2) parallelization is available
- use of symmetry restricted:
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only \(D_{2h}\) and its subgroups can be used for conventional (i.e. not F12) calculations; no symmetry can be used for the F12 methods
Please note that calculations with MP3, MP4, CCSD, BCCD 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.)