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

✍ Scribed by K. Jankowski; J. Gryniaków; K. Rubiniec

• Publisher: John Wiley and Sons
• Year: 1998
• Tongue: English
• Weight: 229 KB
• Volume: 67
• Category: Article
• ISSN: 0020-7608
• DOI: 10.1002/(sici)1097-461x(1998)67:4<221::aid-qua3>3.0.co;2-x

✦ Synopsis

The impact of the choice of molecular orbital sets on the results of the Ž valence-universal coupled cluster method involving up to three-body amplitudes VU-. CCSDT was studied for the H4 model. This model offers a straightforward way of representing all possible symmetry-adapted orbitals. Moreover, the degree of quasidegeneracy of its lowest 1 A states can be varied over a wide range by changing its 1 geometry. Calculations were performed both for 13 sets of standard quantum chemical orbitals and for a vast variety of nonstandard orbital sets defined by nodes of a two-dimensional orbital grid. The performance of various standard orbital sets in VU-CCSDT calculations is compared. It is also documented that for every quasi-degeneracy region there exist nonstandard orbital sets which allow one to obtain more accurate VU-CCSDT energies than the standard orbital sets. In an attempt to provide a general interpretation for some of the alternative orbital sets, we defined a set of orbitals which maximize the proximity of the model and target spaces-maximum proximity orbitals Ž . MPO . It is demonstrated that outside the strong quasi-degeneracy region the energies obtained for the VU-CCSDT approach based on the MPOs are more accurate than for the standard orbital sets.