Second-Order Dispersion Energy Based on Multireference Description of Monomers

Area of Science:

• Quantum Chemistry
• Computational Physics
• Theoretical Chemistry

Background:

• Accurately calculating dispersion energy is crucial for understanding intermolecular forces in weakly interacting systems.
• Multireference systems present significant challenges for traditional quantum chemical methods.
• Existing methods often struggle with the complexity of arbitrary electronic states in these systems.

Purpose of the Study:

• To introduce a novel method for calculating second-order dispersion energy in multireference systems.
• To provide a general and accurate theoretical framework applicable to various electronic states.
• To develop computationally efficient approximations for practical applications.

Main Methods:

• Utilizing response properties derived from extended random phase approximation (RPA) equations.
• Employing one- and two-particle reduced density matrices of monomers.
• Combining the method with generalized valence bond perfect pairing (GVB) or complete active space (CAS) self-consistent field (SCF) descriptions.
• Developing approximations based on Dyall partitioning of monomer Hamiltonians for reduced computational cost.

Main Results:

• The proposed method accurately calculates dispersion energy for model multireference systems (H2···H2 and Be···Be).
• The new method demonstrates superior accuracy compared to single-reference-based approaches for these systems.
• Neither GVB nor CAS descriptions of single-reference monomers improved dispersion energy results over Hartree-Fock.

Conclusions:

• The developed method offers a robust and accurate way to compute dispersion energies for challenging multireference systems.
• The computationally efficient approximations enhance the method's applicability in complex scenarios.
• This work advances the capability to model van der Waals interactions in systems with complex electronic structures.