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Linearized Coupled Cluster Corrections to Antisymmetrized Product of Strongly Orthogonal Geminals: role of dispersive interactions Tamás Zoboki, Ágnes Szabados, and Peter R. Surjan J. Chem. Theory Comput., Just Accepted Manuscript • DOI: 10.1021/ct400138m • Publication Date (Web): 19 Apr 2013 Downloaded from http://pubs.acs.org on April 21, 2013
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Journal of Chemical Theory and Computation
Linearized Coupled Cluster Corrections to Antisymmetrized Product of Strongly Orthogonal Geminals: role of dispersive interactions Tamás Zoboki, Ágnes Szabados, and Péter R. Surján∗ Institute of Chemistry, Laboratory of Theoretical Chemistry, H-1518 Budapest 112, P.O.B. 32 E-mail:
[email protected] Abstract A linearized multi-reference Coupled Cluster (MR-LCC) theory is formulated based on the Antisymmetrized Product of Strongly Orthogonal Geminals (APSG) reference state. The role of dispersive interbond interactions is discussed. The presented theory has lead to qualitatively correct potential curves for the case when both OH bonds dissociate in H2 O, a result that cannot be achieved by adding only perturbative corrections to APSG. The potential curve obtained for the He...He problem practically coincides with the full CI (FCI) result, showing the unexpected accuracy of the MR-LCC approach in this case.
1 Introduction Reference states in quantum chemical calculations are used as zeroth order states obtained at a simpler level to be corrected by perturbative or more sophisticated theories. Coupled Cluster (CC) ∗ To
whom correspondence should be addressed
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theory 1–6 for example offers a powerful tool to describe molecular electronic structure for single reference (SR) problems, i.e. when the electronic wave function can be qualitatively described by a single Slater-determinant. Many chemical processes require the use of two or more determinants even for a qualitative description. Just like Multi-Reference (MR) Perturbation Theory (PT), 7–15 the MR generalization of CC theory is a widely discussed problem, for which several ideas have been proposed, each having their advantages and disadvantages. 16–33 A central problem in MR-CC theory is the construction of permitted excitations and their amplitudes by ensuring the same number of amplitude equations as the number of unknowns, preserving at the same time the commutation of excitation operators. The latter condition is essential to ensure extensivity manifested in the SR case by the ansatz
ˆ
|Ψi = eT |HFi, where |HFi is the SR state (typically the Hartree-Fock determinant) and Tˆ is the excitation operator
occ virt
occ virt
Tˆ = Tˆ1 + Tˆ2 + ... = ∑ ∑ tip p+ i− + ∑ i
p
∑ tipqj p+q+ j−i− + ...
,
(1)
i< j p
