Excited-state methods based on state-averaged long-range CASSCF short-range DFT.

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Theoretical Chemistry

Background:

• Accurate calculation of excited states is crucial for understanding molecular properties and reactions.
• Existing methods like CASSCF-DFT have limitations in accuracy and applicability, especially for complex systems.

Purpose of the Study:

• To develop and evaluate novel state-averaging (SA)-based methodologies for calculating excited states.
• To compare the performance of these new methods against existing approaches for various molecular systems.

Main Methods:

• Proposed two distinct state-averaging (SA)-based methodologies: SA-CAS-srDFT and CI-srDFT.
• Utilized a long-range complete active space self-consistent field (CASSCF) short-range density functional theory (DFT) approach (CAS-srDFT).
• Employed total one-body and on-top pair density (OTPD) for final energy evaluation and benchmarked against multiconfiguration pair-density functional theory (MC-PDFT).

Main Results:

• CI-srDFT provides physically correct potential curves for ethylene, unlike SA-CAS-srDFT.
• CI-srDFT exhibits reduced dependence of excitation energies on the number of states averaged.
• CI-srDFT achieved a mean absolute error of 0.17 eV for singlet excitation energies of organic chromophores using the sr-ctPBE functional.
• The developed methods showed impressive accuracy for organic molecules but were not transferable to transition-metal complexes.

Conclusions:

• CI-srDFT is a more accurate and reliable method for calculating excited states of organic molecules compared to SA-CAS-srDFT.
• Current CASSCF-DFT and MC-PDFT methods do not consistently improve upon CASSCF excitation energies, particularly for transition-metal complexes.