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Int. J. Mol. Sci. 2002, 3(5), 550-569; doi:10.3390/i3050550

Use of a New Cluster Ansatz to Treat Strong Relaxation and Correlation Effects: A Direct Method for Energy Differences

1
Department of Physical Chemistry, Indian Association for the Cultivation of Science, Calcutta 700 032, India
2
Also at Jawaharlal Nehru Centre for Advanced Scientific Research, Bangalore 560 064, India
*
Author to whom correspondence should be addressed.
Received: 11 January 2002 / Accepted: 3 February 2002 / Published: 31 May 2002
(This article belongs to the Special Issue Recent Advances in Coupled Cluster Theory)
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Abstract

We have presented in this paper a new cluster Ansatz for the wave operator for open-shell and/or quasidegenerate states, which takes care of strong relaxation and correlation effects in a compact and efficient manner. This Ansatz allows contraction among the various cluster operators via spectator orbitals, accompanied by suitable combinatorial factors. Since both the orbital and the correlation relaxations are treated on the same footing, it allows us to develop a very useful direct method for energy differences for open shell states relative to a closed-shell ground state, where the total charge for the two states may differ. We have discussed a new spin-free coupled cluster (CC) based direct method and illustrated its performance by evaluating electron affinity of a neutral doublet radical. We have also indicated how the scope of the theory can be extended to compute the state energies of simple open shell configurations as well. In that case, the CC equations terminate after the quartic power of cluster operators – exactly as in the closed-shell situation, which is not the case for the current methods.
Keywords: Valence-universal; valence-specific; strong relaxation and correlation effects; spin-free; coupled cluster; wave operator Valence-universal; valence-specific; strong relaxation and correlation effects; spin-free; coupled cluster; wave operator
This is an open access article distributed under the Creative Commons Attribution License (CC BY 3.0).

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MDPI and ACS Style

Jana, D.; Mahapatra, U.S.; Mukherjee, D. Use of a New Cluster Ansatz to Treat Strong Relaxation and Correlation Effects: A Direct Method for Energy Differences. Int. J. Mol. Sci. 2002, 3, 550-569.

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