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

✍ Scribed by K. Jankowski; L. Meissner; K. Rubiniec

• Publisher: John Wiley and Sons
• Year: 1998
• Tongue: English
• Weight: 190 KB
• Volume: 67
• Category: Article
• ISSN: 0020-7608
• DOI: 10.1002/(sici)1097-461x(1998)67:4<239::aid-qua4>3.0.co;2-w

✦ Synopsis

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 Ž . 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 states obtained for 13 sets of standard quantum chemical 1 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 w demonstrated that the recently defined K. Jankowski et al., Int. J. Quantum Chem. 67, Ž .x Ž . 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.