Linearized Pair-Density Functional Theory.

Area of Science:

• Quantum Chemistry
• Computational Chemistry
• Theoretical Chemistry

Background:

• Multiconfiguration pair-density functional theory (MC-PDFT) is a post-SCF method for calculating ground- and excited-state energies.
• MC-PDFT's single-state nature leads to inaccurate potential energy surface topologies near conical intersections and avoided crossings.

Purpose of the Study:

• Develop a PDFT method that accurately describes potential energy surface topology across the entire nuclear configuration space.
• Enable physically correct *ab initio* molecular dynamics for excited states and Jahn-Teller instabilities.

Main Methods:

• Constructed an effective Hamiltonian operator, the linearized PDFT (L-PDFT) Hamiltonian.
• Expanded the MC-PDFT energy expression to first order in a Taylor series of the wave function density.
• Diagonalized the L-PDFT Hamiltonian to obtain accurate potential energy surfaces.

Main Results:

• L-PDFT successfully recovers correct potential energy surface topology near conical intersections and locally avoided crossings for challenging systems (phenol, methylamine, spiro cation).
• L-PDFT demonstrates superior performance over MC-PDFT and prior multistate PDFT methods in predicting vertical excitations for organic chromophores.

Conclusions:

• L-PDFT provides a significant advancement in describing electronically excited states and molecular dynamics.
• The method accurately captures essential topographical features of potential energy surfaces, crucial for understanding chemical reactivity and spectroscopy.