Excited-state potential energy curves of CH+: a comparison of the EOMCCSDtand full EOMCCSDT results

The recently developed equation-of-motion coupled-cluster (EOMCC) method wi th singles, doubles, and a selected set of triples defined through active o rbitals (EOMCCSDt) is applied to the excited-state potential energy curves of the CH+ ion. The results are compared with the EOMCCSD (EOMCC singles an d doubles), EOMCCSDT (EOMCC singles, doubles, and triples), and full config uration interaction results. It is demonstrated that the EOMCCSDt method pr ovides the excited-state potentials of the EOMCCSDT quality and that the EO MCCSDt and EOMCCSDT methods restore the asymptotic degeneracy of excited st ates, which is broken by the EOMCCSD and other EOMCC doubles models. (C) 20 01 Elsevier Science B.V. All rights reserved.