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Multiconfigurational quantum chemistry: The CASPT2 method

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arxiv 2305.06678 v1 pith:G72JGBIA submitted 2023-05-11 physics.chem-ph physics.comp-phquant-ph

Multiconfigurational quantum chemistry: The CASPT2 method

classification physics.chem-ph physics.comp-phquant-ph
keywords methodcaspt2chapterproblemreferencesometheoryintruder
verification ladder T0 review T1 audit T2 compute T3 formal T4 reserved
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This chapter presents the theory behind the CASPT2 method and its adaptation to a multi-state formalism. The chapter starts with an introduction of the theory of the CASPT2 method - an application of Rayleigh-Schr\"odinger perturbation theory applied to multiconfigurational reference function - as it was originally presented. In particular, we discuss the nature of the reference Hamiltonian and the first-order interacting space. This is followed by some detailed discussion with respect to the intruder state problem and various shift techniques to address this problem. Afterwards a longer review on alternative reference Hamiltonians, which to some degree or completely remove the intruder state problem, is put forward. Subsequently the presently proposed multi-state versions of the CASPT2 method are discussed in some detail. The chapter is concluded with a review of different benchmark assessments of the accuracy of the method and a qualified suggestion on the future development potentials of the approach.

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