Fantastical excited state optimized structures and where to find them

Area of Science:

• Quantum Chemistry
• Computational Spectroscopy
• Theoretical Chemistry

Background:

• Analytic forces for approximate electronic excited states allow exploration of excited state potential energy surfaces (PES).
• This enables computational characterization of excited state minima and saddle points within the Born-Oppenheimer approximation.

Purpose of the Study:

• To investigate the physical reality of computationally optimized excited state minimum structures.
• To identify a diagnostic for non-observable excited state structures in quantum chemistry.

Main Methods:

• Ab initio calculations were performed to analyze excited state potential energy surfaces.
• Simple nonadiabatic dynamics, including a Landau-Zener model, were employed.
• Harmonic frequencies and force constants were evaluated for optimized structures.

Main Results:

• Some excited state minimum structures are 'fantastical,' existing only as computational artifacts.
• These structures display unphysically high harmonic frequencies and force constants.
• Nonadiabatic dynamics show these structures have lifetimes on the order of femtoseconds.

Conclusions:

• High harmonic frequencies serve as a diagnostic for non-observable excited state structures.
• Fantastical structures arise from coupling between electronic states near conical intersections.
• These findings highlight limitations of the Born-Oppenheimer approximation for certain excited states.