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Model study of the impact of orbital choice on the accuracy of coupled‐cluster energies. III. State‐universal coupled‐cluster method
Author(s) -
Jankowski K.,
Meissner L.,
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<239::aid-qua4>3.0.co;2-w
Subject(s) - degeneracy (biology) , atomic orbital , molecular orbital , slater type orbital , coupled cluster , chemistry , cluster (spacecraft) , physics , quantum mechanics , atomic physics , molecular orbital theory , molecule , computer science , bioinformatics , biology , programming language , electron
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 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 1 states obtained for 13 sets of standard quantum chemical 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 demonstrated that the recently defined [K. Jankowski et al., Int. J. Quantum Chem. 67 , 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. © 1998 John Wiley & Sons, Inc. Int J Quant Chem 67: 239–250, 1998