Model study of the impact of orbital choice on the accuracy of coupled-cluster energies. I. Single-reference-state formulation

✍ Scribed by K. Jankowski; K. Kowalski; K. Rubiniec; J. Wasilewski

• Publisher: John Wiley and Sons
• Year: 1998
• Tongue: English
• Weight: 417 KB
• Volume: 67
• Category: Article
• ISSN: 0020-7608
• DOI: 10.1002/(sici)1097-461x(1998)67:4<205::aid-qua2>3.0.co;2-z

✦ Synopsis

The impact of the choice of molecular orbital sets on the results of Ž . single-reference-state coupled-cluster CC methods was studied for the H4 model. This model offers a straightforward way of taking into account all possible symmetry-adapted orbitals. Moreover, the degree of quasi-degeneracy of its ground state can be varied over a wide range by changing its geometry. The CCD, CCSD, and CCSDT approaches are considered. Surfaces representing the dependence of the energy on the parameters defining the orbitals are obtained. It is documented that for every method there exist alternative orbital sets which allow one to obtain more accurate energies than the Ž . standard HF, BO, and NO ones. However, 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 CCD method was found.