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A coupled‐cluster method for quasidegenerate states
Author(s) -
Meissner L.,
Jankowski K.,
Wasilewski J.
Publication year - 1988
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.560340607
Subject(s) - coupled cluster , ansatz , basis set , basis (linear algebra) , cluster (spacecraft) , operator (biology) , nonlinear system , atomic physics , chemistry , physics , mathematics , quantum mechanics , geometry , computer science , molecule , programming language , biochemistry , repressor , transcription factor , gene
A size‐extensive, multireferences coupled‐cluster method for studies of quasidegenerate states based on the Jeziorski–Monkhorst [16] ansatz for the cluster operator (Ω = ∑ e T jP j , where the sum is extended over the configurations spanning the model space), is presented and applied in pilot calculations. The method is referred to as multireference coupled electron‐pair method ( MR CEPM ), because it is assumed that the individual cluster operators can be approximated by their two‐body parts, i.e., T j ≈ T j (2). The linear version of this method ( MR L‐CEPM ) is also discussed. Both methods are applied to two simple model systems: (1) a minimum basis set model involving eight hydrogen atoms in various spacial arrangements for which the degree of quasidegeneracy can be continuously varied; (2) a model involving the C 2ν insertion of Be into H 2 . For the first time in multireference coupled‐cluster calculations, the nonlinear parts of the equations are completely accounted for. The MR CEPM results are very encouraging for strongly quasidegenerate states. The MR L ‐ CEPM results are slightly below the accurate ( FCI ) values.