Constrained Møller–Plesset perturbation theory for charge transfer states

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Abstract
We propose a constrained Møller–Plesset second-order perturbation theory (CMP2) that incorporates electron correlation into the constrained Hartree–Fock (CHF) framework. In CMP2, the CHF solution is employed as the reference state, and a common Lagrange multiplier is introduced for both the zeroth-order and constraint Hamiltonians, enabling the inclusion of correlation effects with minimal modification to existing MP2 implementations. Two schemes are developed for determining the Lagrange multiplier: (i) CMP2-i (iterative), which iteratively updates the multiplier until the MP2 electron density satisfies the constraint, and (ii) CMP2-s (single-shot), which performs a single MP2 calculation using a CHF reference that satisfies the constraint at the Hartree–Fock level. The methods are applied to an intramolecular charge-transfer reaction of the 1,3-dinitrobenzene anion radical. CMP2-s significantly reduces computational cost while reproducing relative energy profiles comparable with CMP2-i. In addition, adiabatic energy profiles obtained with configuration interaction calculations based on CMP2 (CMP2-CI) are in good agreement with those from extended multistate complete active space second-order perturbation theory (XMS-CASPT2), demonstrating that the present approach provides a practical and efficient extension of constrained electronic structure methods to the post-Hartree–Fock level.
Funding Information
  • Japan Society for the Promotion of Science (JP24K01446)
  • Japan Society for the Promotion of Science (JP23K04674)
  • Support for Pioneering Research Initiated by the Next Generation (JPMJSP2124)
  • Ministry of Education, Culture, Sports, Science and Technology (JPMXP1020230325)

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