ISSN:
0020-7608

Keywords:
coupled-cluster theory
;
electron correlation effects
;
molecular orbital choice
;
reference determinant choice
;
quasi-degenerate states
;
Chemistry
;
Theoretical, Physical and Computational Chemistry

Source:
Wiley InterScience Backfile Collection 1832-2000

Topics:
Chemistry and Pharmacology

Notes:
The impact of the choice of molecular orbital sets on the results of 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 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. © 1998 John Wiley & Sons, Inc. Int J Quant Chem 67: 205-219, 1998

Additional Material:
5 Ill.

Type of Medium:
Electronic Resource

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