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Intruder states in multireference perturbation theory: The ground state of manganese dimer
Author(s) -
Camacho Cristopher,
Witek Henryk A.,
Yamamoto Shigeyoshi
Publication year - 2009
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.21074
Subject(s) - complete active space , hamiltonian (control theory) , atomic orbital , basis set , ground state , dimer , perturbation (astronomy) , perturbation theory (quantum mechanics) , chemistry , atomic physics , quantum mechanics , computational chemistry , physics , density functional theory , mathematics , nuclear magnetic resonance , mathematical optimization , electron
Abstract A detailed analysis of a severe intruder state problem in the multistate multireference perturbation theory (MS‐MRPT) calculations on the ground state of manganese dimer is presented. An enormous number of detected intruder states (> 5000) do not permit finding even an approximate shape of the X 1 Σ g +potential energy curve. The intruder states are explicitly demonstrated to originate from quasidegeneracies in the zeroth‐order Hamiltonian spectrum. The electronic configurations responsible for appearance of the quasidegeneracies are identified as single and double excitations from the active orbitals to the external orbitals. It is shown that the quasidegeneracy problem can be completely eliminated using shift techniques despite of its severity. The resultant curves are smooth and continuous. Unfortunately, strong dependence of the spectroscopic parameters of the X 1 Σ g +state on the shift parameter is observed. This finding rises serious controversies regarding validity of employing shift techniques for solving the intruder state problem in MS‐MRPT. Various alternative approaches of removing intruder states (e.g., modification of the basis set or changing the active space) are tested. None of these conventional techniques is able to fully avoid the quasidegeneracies. We believe that the MS‐MRPT calculations on the three lowest A g states of manganese dimer constitute a perfect benchmark case for studying the behavior of MRPT in extreme situations. © 2008 Wiley Periodicals, Inc. J Comput Chem, 2009