Generalization of coupled-cluster response theory to multireference expansion spaces: application of the coupled-cluster singles and doubles effective Hamiltonian |
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Authors: | Seiichiro Ten-no Suehiro Iwata Sourav Pal Debashis Mukherjee |
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Affiliation: | (1) Institute for Molecular Science, Myodaiji, Okazaki 444-8585, Japan, JP;(2) Theoretical Chemistry Group, Physical Chemistry Division, National Chemical Laboratory, Pune 411008, India, IN;(3) Department of Physical Chemistry, Indian Association for the Cultivation of Science, Calcutta 700032, India, IN |
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Abstract: | Employing separate cluster ansatz in time-independent and time-dependent wave-operators, coupled-cluster (CC) response theory is generalized to multireference (MR) expansion spaces. For state energies, this corresponds to the MR secular problem with an arbitrary similarity-transformed effective Hamiltonian, H˜=Ω−1 HΩ. The effective Hamiltonian can be generated via size-extensive CC methods. Thus the states in MR linear response theory (MRLRT) maintain the usual CC core-extensive properties. We have used the Gelfand unitary group basis of the spin-adapted configurations to construct the matrix of H˜ in the MR excitation space. As a preliminary application, the CC singles and doubles effective Hamiltonian is applied to excitation and photoionization energies of the CH+ and N2 molecules, and is compared with experimental results and results from other numerical procedures including conventional CC linear response theory (CC-LRT), MR and full configuration interaction (MRCI and FCI) methods. The numerical results indicate that MRLRT reproduces valence and external excited states quantitatively, combining the best features of CC-LRT and MRCI. Received: 2 July 1998 / Accepted: 28 August 1998 / Published online: 11 November 1998 |
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Keywords: | : Coupled-cluster theory Response theory Effective Hamiltonian Multireference configuration space Excited states Core-extensive |
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