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Reducing CAS‐SDCI space. Using selected spaces in configuration interaction calculations in an efficient way
Author(s) -
PitarchRuiz José,
SánchezMarín José,
Maynau Daniel
Publication year - 2002
Publication title -
journal of computational chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.907
H-Index - 188
eISSN - 1096-987X
pISSN - 0192-8651
DOI - 10.1002/jcc.10092
Subject(s) - configuration interaction , reduction (mathematics) , excited state , space (punctuation) , physics , atomic physics , chemistry , mathematics , computer science , geometry , operating system
A new method is presented, which allows an important reduction of the size of some Configuration Interaction (CI) matrices. Starting from a Complete Active Space (CAS), the numerous configurations that have a small weight in the CAS wave function are eliminated. When excited configurations (e.g., singly and doubly excited) are added to the reference space, the resulting MR‐SDCI space is reduced in the same proportion as compared with the full CAS–SDCI. A set of active orbitals is chosen, but some selection of the most relevant excitations is performed because not all the possible excitations act as SDCI generators. Thanks to a new addressing technique, the computational time is drastically reduced, because the new addressing of the selected active space is as efficient as the addressing of the CAS. The presentation of the method is followed by two test calculations on the N 2 and HCCH molecules. For the N 2 the FCI results are taken as a benchmark reference. The outer valence ionization potentials of HCCH are compared to the experimental values. Both examples allow to test the accuracy of the MR–SDCI compared to that of the corresponding CAS–SDCI, despite the noticeable reduction of the CI space. The algorithm is suitable for the dressing techniques that allow for the correction of the size‐extensivity error. The corrected results are also shown and discussed. © 2002 Wiley Periodicals, Inc. J Comput Chem 23: 1157–1165, 2002