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Use of coupled‐cluster based linear response theory and multireference hermitian MBPT to IP calculations of HF
Author(s) -
Roy S.,
Sengupta S.,
Mukherjee D.,
Mukherjee P. K.
Publication year - 1986
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.560290210
Subject(s) - coupled cluster , ionization , valence (chemistry) , perturbation theory (quantum mechanics) , atomic physics , ionization energy , physics , chemistry , ion , cluster (spacecraft) , hermitian matrix , quantum mechanics , molecule , computer science , programming language
We report in this paper the results of outer and inner valence IP calculations for the HF molecule using two different many‐body methods for the direct evaluation of energy differences. The first is the nonperturbative coupled‐cluster based linear response theory (LRT) and the second is the hermitian open‐shell many‐body perturbation theory (MBPT). A Huzinaga‐Dunning (9 s5p → 5 s 3 p /3 s ) basis has been used. LRT uses an “ionization operator” S as in the equation of motion method (EOM) to generate the ionized states from a coupled‐cluster type of ground state. S is chosen to consist of single ionization and ionization‐cum‐shake‐up operators, thus treating the Koopmans as well as the shake‐up states on equal footing. LRT would thus be capable of computing both the outer and the inner valence regions with equal facility. This is borne out by the results. For the open‐shell MBPT, the model space is chosen to be spanned by the singly ionized determinants. The convergence of the results for the inner valence region is slow, and the results obtained from the [2, 1] Pade' approximants are presented. Unlike the LRT, the inner valence region is not reproduced with full complexity in MBPT, indicating that it is essential to modify the theory by way of expanding the model space to contain the shake‐up determinants also.