CC3 triplet excitation energies using an explicit spin coupled excitation space

Triplet excitation energies are derived in the approximate triples model CC 3 using an explicit spin coupled triplet excitation space. The explicit spi n coupled excitation space gives considerable computational savings compare d to the spin-orbital approach. Sample calculations are performed on the Be -2 and CH2 systems and the performance of the CC3 results are evaluated fro m a comparison with full configuration interaction (FCI) results. (C) 2001 American Institute of Physics.