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  An alternative choice of the zeroth-order Hamiltonian in CASPT2 theory

Kollmar, C., Sivalingam, K., & Neese, F. (2020). An alternative choice of the zeroth-order Hamiltonian in CASPT2 theory. The Journal of Chemical Physics, 152(21): 214110. doi:10.1063/5.0010019.

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 Creators:
Kollmar, Christian1, Author              
Sivalingam, Kantharuban2, Author              
Neese, Frank1, Author              
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1Research Department Neese, Max-Planck-Institut für Kohlenforschung, Max Planck Society, ou_2541710              
2Research Group Wennmohs, Max-Planck-Institut für Kohlenforschung, Max Planck Society, ou_2541706              

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 Abstract: A zeroth-order Hamiltonian based on Koopmans matrices for complete active space second-order perturbation theory (CASPT2) is presented. This Hamiltonian involves three types of Fock matrices. The original CASPT2 Fock matrix is retained for excitation classes where the excitation does not change the number of electrons in the complete active space (CAS). For excitation classes involving a change in the number of electrons in the CAS, two alternative Fock matrices corresponding to either positive or negative ionization of the CAS are introduced. These are constructed such that they exactly reproduce the Koopmans matrices for a singly ionized CAS. Test calculations indicate that the method gives better excitation energies than CASPT2 without using empirical parameters, for example, the ionization potential-electron affinity shift, which is also designed to improve excitation energies. The method is also less prone to intruder states than conventional CASPT2. Moreover, the dissociation curve for the chromium dimer looks much more reasonable than the one obtained with conventional CASPT2.

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Language(s): eng - English
 Dates: 2020-04-072020-05-152020-06-042020-06-07
 Publication Status: Published in print
 Pages: 10
 Publishing info: -
 Table of Contents: -
 Rev. Type: Peer
 Identifiers: DOI: 10.1063/5.0010019
 Degree: -

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Title: The Journal of Chemical Physics
  Other : J. Chem. Phys.
Source Genre: Journal
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Publ. Info: Woodbury, N.Y. : American Institute of Physics
Pages: - Volume / Issue: 152 (21) Sequence Number: 214110 Start / End Page: - Identifier: ISSN: 0021-9606
CoNE: https://pure.mpg.de/cone/journals/resource/954922836226