The active-space equation-of-motion coupled-cluster methods for excited electronic states: The EOMCCSDt approach

The idea of selecting the most important higher-than-doubly excited configu rations in single-reference coupled-cluster (CC) calculations for quasidege nerate ground states of molecular systems through the use of active orbital s is extended to excited electronic states via the equation-of-motion (EOM) CC formalism. The resulting EOMCCSDt method, in which triexcited clusters T-3 and the corresponding three-body components of the EOMCC excitation ope rator R are restricted to internal and semiinternal components defined thro ugh active orbitals, is capable of significantly improving the vertical exc itation energies obtained with the conventional EOMCCSD (EOMCC singles and doubles) approach at a fraction of the computer cost associated with the fu ll EOMCCSDT (EOMCC singles, doubles, and triples) calculations. The results of pilot calculations for the H-8, CH2, and CH+ molecules indicate that th e EOMCCSDt method using small active spaces is as accurate as the EOMCCSDT approach. In particular, the EOMCCSDt method is capable of accurately descr ibing states that are doubly excited relative to the reference state. (C) 2 000 American Institute of Physics. [S0021-9606(00)30143-X].