Analytical gradients for nuclear-electronic orbital multistate density functional theory: Geometry optimizations and...

Area of Science:

• Quantum Chemistry
• Theoretical Chemistry
• Chemical Dynamics

Background:

• Hydrogen tunneling is crucial for biological and chemical processes.
• The nuclear-electronic orbital multistate density functional theory (NEO-MSDFT) method describes quantum hydrogen transfer.
• Previous work demonstrated NEO-MSDFT's accuracy for fixed molecular systems.

Purpose of the Study:

• Derive and implement NEO-MSDFT analytical gradients for ground and excited vibronic states.
• Optimize geometries and characterize transition states for proton transfer reactions.
• Generate minimum energy paths (MEPs) to study quantum proton transfer.

Main Methods:

• Implemented NEO-MSDFT analytical gradients and semi-numerical Hessians.
• Optimized equilibrium and transition state geometries.
• Generated minimum energy paths (MEPs) for proton transfer in deprotonated acetylene dimer and malonaldehyde.

Main Results:

• NEO-MSDFT analytical gradients were successfully derived and implemented.
• Proton transfer barriers were lower due to inherent zero-point energy in the quantized proton.
• Proton densities showed symmetric or asymmetric bilobal character, indicating hydrogen tunneling.
• Analysis revealed C-O bond length changes drive proton transfer in malonaldehyde.

Conclusions:

• The implemented NEO-MSDFT gradients provide a foundation for future reaction path studies.
• This method enables direct nonadiabatic dynamics simulations for hydrogen transfer reactions.
• Quantum mechanical treatment of protons significantly impacts reaction pathways and energetics.