Constrained Møller-Plesset perturbation theory for charge transfer states.

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Theoretical Chemistry

Background:

• Constrained Hartree-Fock (CHF) provides a framework for electronic structure calculations with specific constraints.
• Incorporating electron correlation beyond the Hartree-Fock level is crucial for accurate chemical predictions.
• Existing methods may require significant modifications for constrained calculations.

Purpose of the Study:

• To develop a constrained Møller-Plesset second-order perturbation theory (CMP2) method.
• To integrate electron correlation effects into the CHF framework efficiently.
• To provide a practical extension of constrained electronic structure methods to the post-Hartree-Fock level.

Main Methods:

• Constrained Møller-Plesset second-order perturbation theory (CMP2) using CHF as the reference state.
• Introduction of a common Lagrange multiplier for zeroth-order and constraint Hamiltonians.
• Two schemes for multiplier determination: iterative (CMP2-i) and single-shot (CMP2-s).
• Application to an intramolecular charge-transfer reaction of 1,3-dinitrobenzene anion radical.
• Configuration interaction (CMP2-CI) and extended multistate complete active space second-order perturbation theory (XMS-CASPT2) for energy profile analysis.

Main Results:

• CMP2 successfully incorporates electron correlation into the CHF framework with minimal modifications.
• CMP2-s offers a significant reduction in computational cost compared to CMP2-i.
• Both CMP2 schemes reproduce relative energy profiles comparable to each other.
• CMP2-CI adiabatic energy profiles show good agreement with XMS-CASPT2 results.

Conclusions:

• The proposed CMP2 method is a practical and efficient extension of constrained electronic structure theory.
• CMP2 enables accurate post-Hartree-Fock calculations for systems requiring constraints.
• The CMP2-s scheme provides a computationally advantageous approach for incorporating electron correlation.