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Model study of the impact of orbital choice on the accuracy of coupled‐cluster energies. II. Valence‐universal coupled‐cluster method
Author(s) -
Jankowski K.,
Gryniaków J.,
Rubiniec K.
Publication year - 1998
Publication title -
international journal of quantum chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.484
H-Index - 105
eISSN - 1097-461X
pISSN - 0020-7608
DOI - 10.1002/(sici)1097-461x(1998)67:4<221::aid-qua3>3.0.co;2-x
Subject(s) - atomic orbital , degeneracy (biology) , coupled cluster , physics , valence (chemistry) , molecular orbital , slater type orbital , valence bond theory , basis set , orbital overlap , chemistry , atomic physics , quantum mechanics , molecule , bioinformatics , biology , electron
The impact of the choice of molecular orbital sets on the results of the 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 quasi‐degeneracy of its lowest 1 A 1 states can be varied over a wide range by changing its 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 (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. © 1998 John Wiley & Sons, Inc. Int J Quant Chem 67: 221–237, 1998