Dependence of state-universal coupled-cluster energies on the model-space-defining orbitals for states of varying quasidegeneracy

A new version of the state-universal coupled-cluster (SU-CC) method, termed MRCCSD-4 method, has been developed and applied in investigations of the e ffect of the choice of the model-space-determining orbitals on the energies corresponding to states of variable quasidegeneracy. The present version t akes into account one- and two-electron excitations (SU-CCSD) and is based on the most complete equations for the cluster amplitudes and effective Ham iltonian used so far in the literature. Calculations were performed for the H4 model which offers a straightforward way of defining any symmetry-adapt ed orbitals as well as the possibility of varying the degree of quasidegene racy. Energies of the two lowest (1)A(1) states were calculated for a vast variety of orbitals including nonstandard ones. It is documented that there exist nonstandard orbital sets which allow one to obtain more accurate SU- CCSD energies than the standard quantum-chemical orbitals. Among these, of special interest are orbitals representing generalizations of the Brueckner orbitals to the case of multi-dimensional model spaces. It is demonstrated that for intermediate and weak quasidegeneracy the impact of the approxima tions defining various versions of the SU-CCSD method can be minimized by a n adequate choice of the orbital sets. (C) 2001 Elsevier Science B.V. All r ights reserved.