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Multiple solutions of the single‐reference coupled‐cluster equations. I. H4 model revisited
Author(s) -
Jankowski K.,
Kowalski K.,
Jankowski P.
Publication year - 1994
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.560500504
Subject(s) - cluster (spacecraft) , simple (philosophy) , nonlinear system , operator (biology) , coupled cluster , excited state , state (computer science) , statistical physics , mathematics , physics , chemistry , computer science , quantum mechanics , algorithm , molecule , transcription factor , gene , programming language , philosophy , biochemistry , epistemology , repressor
Single‐reference coupled‐cluster methods corresponding to various approximate forms of the cluster operator including up to four‐electron operators are applied to the simple H 4 model system with the aim of numerically studying the attainability and properties of multiple solutions of the systems of nonlinear equations for the cluster amplitudes. Various solutions have been obtained for the excited states 2 1 A 1 and 5 1 A 1 . It is documented that there exist nonphysical nonstandard solutions that yield energies very close to the physical ones. A simple way of selecting the physically meaningful solutions is proposed. It is demonstrated that the attainability of nonstandard solutions describing a state of H4 within the framework of a given SE ‐ CC method strongly depends on the cluster structure of that state. © 1994 John Wiley & Sons, Inc.