MODEL STUDY OF THE IMPACT OF ORBITAL CHOICE ON THE ACCURACY OF COUPLED-CLUSTER ENERGIES - I - SINGLE-REFERENCE-STATE FORMULATION

The impact of the choice of molecular orbital sets on the results of s ingle-reference-state coupled-cluster (CC) methods was studied for the H4 model. This model offers a straightforward way of taking into acco unt all possible symmetry-adapted orbitals. Moreover, the degree of qu asi-degeneracy of its ground state can be varied over a wide range by changing its geometry. The CCD, CCSD, and CCSDT approaches are conside red. Surfaces representing the dependence of the energy on the paramet ers defining the orbitals are obtained. It is documented that for ever y method there exist alternative orbital sets which allow one to obtai n more accurate energies than the standard (HF, BO, and NO) ones. Howe ver, for many of the former orbital sets, one obtains relatively large one-body amplitudes or one may encounter problems with solving the CC equations by conventional methods. An interesting variety of orbitals which might be useful for studies of quasi-degenerate states by the C CD method was found. (C) 1998 John Wiley & Sons, Inc.