Generalized Formulation of the Density Functional Tight Binding-Based Restricted Ensemble Kohn-Sham Method with...

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Theoretical Chemistry

Background:

• Accurate modeling of excited states and conical intersections is vital for understanding photochemical processes.
• Existing methods like DFTB have limitations in describing multireference systems.

Purpose of the Study:

• To generalize the density functional tight binding (DFTB) approach by incorporating the state-interaction state-average spin-restricted ensemble-referenced Kohn-Sham (SI-SA-REKS or SSR) method.
• To include onsite correction (OC) and long-range corrected (LC) functionals for improved accuracy.
• To accurately describe conical intersections and excited-state dynamics.

Main Methods:

• Generalized formulation of LC-OC-DFTB/SSR method.
• Benchmarking against various DFTB calculation methods for excitation energies and conical intersection structures.
• Performing excited-state molecular dynamics simulations using the developed method.

Main Results:

• The LC-OC-DFTB/SSR method provides more accurate energies and analytic gradients for individual microstates.
• The multireference character of SSR correctly describes conical intersections.
• The onsite correction (OC) contribution to the LC functional is essential for obtaining accurate conical intersection geometries.

Conclusions:

• The developed LC-OC-DFTB/SSR method offers a significant improvement for calculating excited-state properties and conical intersections.
• This method is crucial for accurate molecular dynamics simulations of photochemical systems, such as molecular rotary motors.