PERTURBATIVE TREATMENT OF THE SIMILARITY TRANSFORMED HAMILTONIAN IN EQUATION-OF-MOTION COUPLED-CLUSTER APPROXIMATIONS

A series of size-consistent approximations to the equation-of-motion c oupled cluster method in the singles and doubles approximation (EOM-CC SD) are developed by subjecting the similarity transformed Hamiltonian ($) over bar H=exp(-T)H exp(T) to a perturbation expansion. Attention is directed to N and N-1 electron final state realizations of the met hod defined by truncation of H at second order. Explicit spin-orbital equations for the energy and its first derivative are documented for b oth approaches [EOMEE-CCSD(2) and EOMIP-CCSD(2), respectively], and ha ve been implemented in a large-scale quantum chemistry pro,sram. Verti cal ionization potentials calculated by EOMIP-CCSD(2) are shown to be equivalent to those of an approach presented recently by Nooijen and S nijders [J. Chem. Phys. 102, 1681 (1995)]. Applications of both EOMIP- CCSD(2) and EOMEE-CCSD(2) provide results for final state properties t hat compare favorably with these obtained in full EOM-CCSD calculation s. Analysis of the computational aspects of the approximate and full E OM-CCSD methods shows that the cost of EOMIP-CCSD(2) energy and gradie nt calculations scales in proportion to the fifth power of the basis s et size, a significant savings over the sixth power dependence of EOMI P-CCSD. This feature is of great practical importance, as it shows tha t this N-1 electron final state approach has a large domain of applica bility and is therefore likely to become a valuable tool for applicati on calculations. On the other hand, the same cannot be said for EOMEE- CCSD(2) since its asymptotic computational dependence and storage requ irements are the same as the full EOMEE-CCSD method. (C) 1995 American Institute of Physics.