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American Institute of Physics, The Journal of Chemical Physics, 21(134), p. 214116

DOI: 10.1063/1.3592494

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A state-specific partially internally contracted multireference coupled cluster approach

Journal article published in 2011 by Dipayan Datta ORCID, Liguo Kong, Marcel Nooijen
This paper was not found in any repository, but could be made available legally by the author.
This paper was not found in any repository, but could be made available legally by the author.

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Abstract

A state-specific partially internally contracted multireference coupled cluster approach is presented for general complete active spaces with arbitrary number of active electrons. The dominant dynamical correlation is included via an exponential parametrization of internally contracted cluster operators (\documentclass[12pt]{minimal}\begin{document}$\hat{T}$\end{document}T̂) which excite electrons from a multideterminantal reference function. The remaining dynamical correlation and relaxation effects are included via a diagonalization of the transformed Hamiltonian \documentclass[12pt]{minimal}\begin{document}${\smash{\hat{\overline{H}}=e^{-\hat{T}}\hat{H}e^{\hat{T}}}}$\end{document}H¯̂=e−T̂ĤeT̂ in the multireference configuration interaction singles space in an uncontracted fashion. A new set of residual equations for determining the internally contracted cluster amplitudes is proposed. The second quantized matrix elements of \documentclass[12pt]{minimal}\begin{document}${\smash{\hat{\overline{H}}}}$\end{document}H¯̂, expressed using the extended normal ordering of Kutzelnigg and Mukherjee, are used as the residual equations without projection onto the excited configurations. These residual equations, referred to as the many-body residuals, do not have any near-singularity and thus, should allow one to solve all the amplitudes without discarding any. There are some relatively minor remaining convergence issues that may arise from an attempt to solve all the amplitudes and an initial analysis is provided in this paper. Applications to the bond-stretching potential energy surfaces for N2, CO, and the low-lying electronic states of C2 indicate clear improvements of the results using the many-body residuals over the conventional projected residual equations.