Target State Optimized Density Functional Theory for Electronic Excited and Diabatic States

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Theoretical Chemistry

Background:

• Accurate exploration of electronic excited configurations and localized diabatic states is crucial for understanding molecular properties and reactions.
• Conventional methods like delta self-consistent field (ΔSCF) can suffer from state collapse and lack flexibility in defining target states.
• Time-dependent density functional theory (TD-DFT) is widely used but has limitations for certain types of electronic excitations.

Purpose of the Study:

• To introduce and describe a flexible self-consistent field method, Target State Optimization (TSO), for exploring electronic excited configurations and localized diabatic states.
• To demonstrate the applicability and advantages of TSO within Hartree-Fock and Kohn-Sham density functional theory (DFT) frameworks.
• To provide a robust computational tool for analyzing intermolecular interactions and simulating charge transfer processes.

Main Methods:

• The Target State Optimization (TSO) method partitions molecular orbitals into distinct subspaces based on excitation or localization patterns.
• Orbital-subspace constraints prevent mixing between different subspaces, enabling variational optimization of determinant wave functions.
• TSO is implemented in both Hartree-Fock and Kohn-Sham DFT (TSO-DFT), including density projection procedures and working equations.

Main Results:

• TSO-DFT and TD-DFT yield comparable excitation energies for valence excited states of organic compounds (average errors of 0.5 and 0.4 eV, respectively).
• TSO-DFT demonstrates superior performance over conventional TD-DFT for core excitations, doubly excited states, and charge-transfer states.
• TSO-DFT enables the definition of variationally optimized charge-localized diabatic states for energy decomposition analysis and molecular dynamics simulations.

Conclusions:

• The TSO method provides a flexible and robust approach for studying complex electronic excited states and localized diabatic states.
• TSO-DFT offers improved accuracy and reliability compared to TD-DFT for specific challenging electronic configurations.
• The method facilitates deeper insights into intermolecular interactions and enables advanced simulations of charge transfer phenomena.