Compressed-State Multistate Pair-Density Functional Theory.

Area of Science:

• Quantum Chemistry
• Computational Chemistry
• Theoretical Chemistry

Background:

• Multiconfiguration pair-density functional theory (MC-PDFT) is a multireference method for calculating excited states.
• MC-PDFT exhibits incorrect potential energy surface topology at conical intersections due to its final computational step.
• Previous methods like VMS-PDFT and XMS-PDFT aimed to correct this by diagonalizing an effective Hamiltonian matrix.

Purpose of the Study:

• To develop a new computational method that is both efficient and accurate for calculating excited states, specifically addressing the topological errors at conical intersections.
• To overcome the limitations of existing methods, such as the computational cost of VMS-PDFT and the accuracy issues of XMS-PDFT.

Main Methods:

• A novel method, compressed-state multistate PDFT (CMS-PDFT), was developed.
• CMS-PDFT involves diagonalizing a model-space effective Hamiltonian matrix.
• The method optimizes intermediate states by maximizing the trace of the classical Coulomb energy, leading to compressed electron densities.

Main Results:

• CMS-PDFT demonstrates efficiency comparable to XMS-PDFT.
• CMS-PDFT achieves accuracy on par with VMS-PDFT.
• The new CMS-PDFT method shows robust performance, even in cases where XMS-PDFT fails.

Conclusions:

• CMS-PDFT offers a significant advancement in calculating excited states, providing a robust and accurate approach.
• The method effectively resolves the topological inaccuracies of MC-PDFT at conical intersections.
• CMS-PDFT presents a computationally viable alternative for complex quantum chemistry problems.