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Multireference coupled‐cluster methods for ground and low‐lying excited states. A benchmark illustration on CH + potentials
Author(s) -
Li Xiangzhu,
Paldus Josef
Publication year - 2010
Publication title -
international journal of quantum chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.484
H-Index - 105
eISSN - 1097-461X
pISSN - 0020-7608
DOI - 10.1002/qua.22761
Subject(s) - coupled cluster , excited state , complete active space , atomic physics , singlet state , configuration interaction , ab initio , chemistry , full configuration interaction , cluster (spacecraft) , amplitude , physics , computational chemistry , quantum mechanics , molecule , density functional theory , basis set , computer science , programming language
Multireference (MR), general‐model‐space (GMS), state‐universal (SU) coupled‐cluster (CC) method that considers singly (S) and doubly (D) excited cluster amplitudes relative to the reference configurations spanning the model space (GMS SU CCSD), as well as its externally corrected (ec) version ( N , M )‐CCSD that uses N ‐reference MR CISD as an external source of higher‐than‐pair cluster amplitudes in a M ‐reference GMS CCSD, are used to investigate low‐lying states of the CH + ion. Relying on a simple ab initio model that enables a comparison with the exact full configuration interaction energies, the performance of the GMS‐based methods is assessed in the whole relevant range of internuclear separations. It is shown that the ec ( N , M )‐CCSD version provides best results for both the singlet and the triplet states considered. For triplets, the use of high‐spin ( M S = 1) references is to be preferred. The GMS‐based MR SU CC results for the ten low‐lying states of CH + clearly indicate the usefulness and reliability of these approaches. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2010