Model study of the impact of orbital choice on the accuracy of coupled-cluster energies. III. State-universal coupled-cluster method

Model studies of the impact of the choice of molecular orbital sets on the accuracy of the results of the state-universal coupled-cluster method involving one-and two-body excitations (SU-CCSD) were performed for the H4 model, which offers a straightforward way of representing any symmetry-adapted orbitals as well as the possibility of varying over a wide range the degree of quasi-degeneracy of a state. Energies of the three lowest (1)A(1) states obtained for 13 sets of standard quantum chemical orbitals as well as for a vast variety of nonstandard orbital sets defined by nodes of a two-dimensional grid are compared. It is shown that there exist nonstandard orbital sets that allow one to obtain more accurate energies than the standard orbital sets. It is also demonstrated that the recently defined [K. Jankowski et al., Int. J. Quantum Chem. 67, 221 (1998)] maximum proximity orbitals (MPO) yield more accurate results than any other of the commonly applied orbital sets. These orbitals are especially effective outside the strong-quasi-degeneracy region. (C) 1998 John Wiley & Sons, Inc.