In this work, we generalize the recently proposed matrix product state perturbation theory (MPSPT) for calculating energies of excited states using quasi-degenerate (QD) perturbation theory. Our formulation uses the Kirtman-Certain-Hirschfelder canonical Van Vleck perturbation theory, which gives Hermitian effective Hamiltonians at each order, and also allows one to make use of Wigner’s 2n + 1 rule. Further, our formulation satisfies Granovsky’s requirement of model space invariance which is important for obtaining smooth potential energy curves. Thus, when we use MPSPT with the Dyall Hamiltonian, we obtain a model space invariant version of quasi-degenerate n-electron valence state perturbation theory (NEVPT), a property that the usual formulation of QD-NEVPT2 based on a multipartitioning technique lacked. We use our method on the benchmark problems of bond breaking of LiF which shows ionic to covalent curve crossing and the twist around the double bond of ethylene where significant valence-Rydberg mixing occurs in the excited states. In accordance with our previous work, we find that multi-reference linearized coupled cluster theory is more accurate than other multi-reference theories of similar cost.
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21 January 2016
Research Article|
January 19 2016
Quasi-degenerate perturbation theory using matrix product states
Sandeep Sharma;
Sandeep Sharma
a)
1
Max Planck Institute for Solid State Research
, Heisenbergstraße 1, 70569 Stuttgart, Germany
Search for other works by this author on:
Guillaume Jeanmairet;
Guillaume Jeanmairet
1
Max Planck Institute for Solid State Research
, Heisenbergstraße 1, 70569 Stuttgart, Germany
Search for other works by this author on:
a)
Electronic address: sanshar@gmail.com
b)
Electronic address: a.alavi@fkf.mpg.de
J. Chem. Phys. 144, 034103 (2016)
Article history
Received:
October 24 2015
Accepted:
December 29 2015
Citation
Sandeep Sharma, Guillaume Jeanmairet, Ali Alavi; Quasi-degenerate perturbation theory using matrix product states. J. Chem. Phys. 21 January 2016; 144 (3): 034103. https://doi.org/10.1063/1.4939752
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