Implementation of electronic ground states and singlet and triplet excitation energies in coupled cluster theory with approximate triples corrections

An implementation of triples corrections for the calculation of the electronic ground states and for singlet and triplet excitation energies within the CC3 model is discussed. At most objects of size (VO2)-O-2 and (VO)-O-3 are kept in memory and on disc, respectively (V is the number of virtual orbital and O is the number of occupied orbitals). The used strategy means that more terms that scales as (VO3)-O-4 has to be calculated than if the triples amplitudes are kept on disc but it allows larger cases to be handled. Sample calculations are presented for the triplet excitation energies of benzene. (C) 2002 American Institute of Physics