A new size extensive multireference perturbation theory.

Area of Science:

• Quantum Chemistry
• Theoretical Chemistry
• Computational Chemistry

Background:

• Accurate prediction of molecular properties is crucial in chemistry.
• Existing multireference methods can be computationally expensive.
• Need for efficient and accurate theoretical models for complex systems.

Purpose of the Study:

• To develop a new, computationally efficient multireference perturbation series.
• To assess the accuracy and size-extensivity of the new method.
• To compare its performance against established theoretical approaches.

Main Methods:

• Derivation of a novel multireference perturbation series based on Rayleigh-Schrödinger perturbation theory.
• Numerical validation using supermolecules (H2 and BH monomers).
• Application to calculate spectroscopic constants and ground state energies for various molecules (F2, Cl2, C2-, B2, C2+, H2O, NH2, CH2, CH4, HF).

Main Results:

• The new method is orbitally invariant and computationally comparable to single-reference Møller-Plesset perturbation theory.
• Demonstrated size-extensivity for second- and third-order energies.
• Accurate calculations of spectroscopic constants and ground state energies.
• Reliable investigation of dissociation behaviors.

Conclusions:

• The developed multireference perturbation series provides a computationally feasible and accurate alternative for electronic structure calculations.
• The method shows excellent performance for various molecular systems, including bond breaking and formation.
• It offers a promising tool for advancing theoretical chemistry research.