High-order determinantal equation-of-motion coupled-cluster calculations for electronic excited states

A general-order equation-of-motion coupled-cluster (EOM-CC) method, which i s capable of computing the excitation energies of molecules at any given pa ir of orders (m and n) of the cluster operator and the linear excitation op erator, is developed by employing a determinantal algorithm. The EOM-CC(m,n ) results of the vertical excitation energies are presented for CH+ with m and n varied independently in the range of 1 less than or equal to m,n less than or equal to 4 and for CH2 with 1 less than or equal to m = n less tha n or equal to 6. EOM-CCSDT [EOM-CC(3,3)] provides the excitation energies t hat are within 0.1 eV of the full configuration interaction results for dom inant double replacement transitions. (C) 2000 Elsevier Science B.V. All ri ghts reserved.