Multireference Rayleigh-Schrödinger perturbation theory and its application.

Area of Science:

• Quantum Chemistry
• Theoretical Chemistry
• Computational Chemistry

Background:

• Accurate prediction of molecular properties is crucial in chemistry.
• Existing theoretical models face challenges in describing complex electronic structures.
• Multireference methods are essential for systems with strong electron correlation.

Purpose of the Study:

• To derive and validate a new multireference Rayleigh-Schrödinger perturbation theory (MRSPT).
• To optimize the unknown parameter ε¯ at the MRSPT2 level for improved accuracy.
• To assess the size extensivity and applicability of the developed MRSPT.

Main Methods:

• Derivation of MRSPT based on complete active space multireference wavefunction.
• Optimization of the active orbital energy parameter ε¯ at the MRSPT2 level.
• Numerical demonstration of size extensivity for second and third order theories.

Main Results:

• MRSPT2 and MRSPT3 were shown to be numerically size extensive.
• Ground state energies were computed for F₂, AlH, HCl, and P₂.
• Equilibrium bond lengths and harmonic vibrational frequencies were optimized for BH, BF, P₂, HF, and F₂.
• Dissociation behaviors of NH₃ and OH⁻ were investigated.

Conclusions:

• The developed MRSPT provides accurate results for molecular properties.
• The theory demonstrates advantages compared to other theoretical models and experimental data.
• MRSPT offers a promising approach for studying complex chemical systems.