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Multiple solutions of the single‐reference coupled‐cluster equations. II. Alternative reference states
Author(s) -
Jankowski K.,
Kowalski K.,
Jankowski P.
Publication year - 1995
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/qua.560530507
Subject(s) - degeneracy (biology) , excited state , coupled cluster , cluster (spacecraft) , nonlinear system , state (computer science) , range (aeronautics) , ground state , statistical physics , yield (engineering) , simple (philosophy) , mathematics , physics , atomic physics , quantum mechanics , thermodynamics , computer science , algorithm , molecule , materials science , bioinformatics , philosophy , epistemology , composite material , biology , programming language
Single‐reference coupled‐cluster ( SR – CC ) methods parametrized with respect to four alternative reference‐state configurations and orbital sets are applied to the simple H4 model system in which the degree of quasi‐degeneracy of the electronic states can be varied in a wide range. Both the ground state and 10 excited states are considered with the aim of numerically studying the attainability and properties of multiple solutions of the system of nonlinear equations for the cluster amplitudes. Comparisons of up to four alternative descriptions of a given state in terms of various solutions of the SR – CC equations are made. It is found that for some excited states different parametrizations yield very close results and that the classification of the solutions into standard and nonstandard ones may not be straightforward. © 1995 John Wiley & Sons, Inc.