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arXiv · 2007.06125

Localized and delocalized states of a diamine cation: A critical test of wave function methods

M. Gałyńska·V. Ásgeirsson·H. Jónsson·R. Björnsson
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Abstract

The relative stability of a localized and delocalized electronic state in the same molecule, the N,N' -Dimethylpiperazine cation, is calculated at various levels of theory up to multireference configuration interaction (MRCI+Q). This system has received a great deal of attention because of recent experimental studies of corresponding Rydberg states of the molecule and the failure of most density functional approximations to produce a metastable localized state. A cut through the energy surface involving two dihedral angles is generated at the level of MRCI+Q as well as Hartree-Fock (HF), Möller-Plesset second order perturbation theory (MP2), coupled cluster theory with and without perturbative triple excitations (CCSD and CCSD(T)) and complete active space self-consistent field calculations with and without perturbative correction (CASSCF and NEVPT2). Remarkably, while CCSD produces a localized state, CCSD(T) does not, and similarly, large active-space CASSCF does while NEVPT2 does not. The inclusion of dynamic correlation in a perturbative way thus adversely affects the accuracy of the calculation, most notably for CCSD(T), the 'golden standard'. The MRCI+Q results are in close correspondence with the experimental results, as well as CAS(19,20) DMRG-CASSCF calculations. The results presented here establish a benchmark system for the study of electronic state localization.

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M. Gałyńska, V. Ásgeirsson, H. Jónsson, R. Björnsson. 2020-07-12. Localized and delocalized states of a diamine cation: A critical test of wave function methods. https://arxiv.org/abs/2007.06125

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