Toward more accurate adiabatic connection approach for multireference wavefunctions.

Area of Science:

• Quantum Chemistry
• Computational Physics
• Theoretical Chemistry

Background:

• Multiconfigurational adiabatic connection (AC) formalism is key for dynamic correlation in quantum chemistry.
• Current AC methods often rely on approximations like fixed one- and two-electron reduced density matrices (1- and 2-RDMs) and extended random phase approximation (ERPA).

Purpose of the Study:

• To investigate the impact of removing the "fixed-RDM" approximation in AC calculations.
• To assess the performance of AC models with exact versus approximated RDMs.

Main Methods:

• Utilized two electronic Hamiltonian partitionings: group product function and Dyall Hamiltonians.
• Generated exact AC integrands using Density Matrix Renormalization Group (DMRG) full configuration interaction.
• Investigated two AC models: one with exact 1- and 2-RDMs, another with second-order expansions in the coupling constant within ERPA equations.

Main Results:

• Calculations were performed on model molecules to evaluate the effect of the "fixed-RDM" approximation.
• Compared results from AC models using exact RDMs against those using approximated RDMs.

Conclusions:

• Lifting the "fixed-RDM" approximation presents a promising strategy for enhancing the accuracy of existing AC approximations.
• This approach offers a viable path toward more precise dynamic correlation calculations in quantum chemistry.