Analytical Gradients for Nuclear-Electronic Orbital Time-Dependent Density Functional Theory: Excited-State Geometry...

Area of Science:

• Quantum Chemistry
• Computational Chemistry
• Photochemistry

Background:

• Computational investigations of photochemical processes require accurate excited-state calculations.
• Conventional methods often neglect nuclear quantum effects and non-Born-Oppenheimer behavior.

Purpose of the Study:

• To derive and implement analytical gradients for the nuclear-electronic orbital time-dependent density functional theory (NEO-TDDFT) method.
• To provide programmable equations for analytical gradients and the NEO-DFT analytical Hessian.
• To enable more accurate quantum chemical calculations of ground and excited states.

Main Methods:

• Developed analytical gradients for NEO-TDDFT and its Tamm-Dancoff approximation (NEO-TDA).
• Implemented equations for NEO-DFT analytical Hessian.
• Applied the nuclear-electronic orbital (NEO) approach to include anharmonic zero-point energy (ZPE) and vibronic mixing.

Main Results:

• Computed 0-0 adiabatic excitation energies for nine small molecules with quantized protons, showing slight improvement over conventional methods.
• Performed excited-state geometry optimizations for two intramolecular proton-transfer systems.
• NEO calculations revealed stronger intramolecular hydrogen bonds in excited states.

Conclusions:

• The developed NEO-TDDFT gradients provide a foundation for nonadiabatic dynamics simulations.
• This approach enhances the accuracy of calculations involving quantum protons and proton transfer.
• The study facilitates better understanding of fundamental photochemical processes.