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Spin‐unrestricted calculations in quantum chemistry
Author(s) -
CassamChenaï P.,
Chandler G. S.
Publication year - 1993
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.560460502
Subject(s) - eigenfunction , spin (aerodynamics) , fock space , wave function , space (punctuation) , function (biology) , chemistry , complete active space , operator (biology) , quantum mechanics , field (mathematics) , physics , computational chemistry , density functional theory , eigenvalues and eigenvectors , mathematics , basis set , transcription factor , pure mathematics , thermodynamics , linguistics , philosophy , biochemistry , repressor , evolutionary biology , biology , gene
The unrestricted complete active space self‐consistent field ( UCASSCF ) function is defined, and a proof that a UCASSCF eigenfunction of the spin operator S 2 is a CASSCF function is given. The spin‐contamination for an unrestricted Hartree–Fock ( UHF ) function is evaluated by using Araki angle operators, and the UHF function is then projected on the restricted open‐shell Hartree–Fock ( ROHF ) space. The present analysis has deep consequences since it implies that the only non‐spin‐contaminated UHF functions are the ROHF functions. This is illustrated in a calculation of the spin density of He 2 + . © 1993 John Wiley & Sons, Inc.