Analytic Nuclear Gradients for Complex Potential Energy Surfaces: A Projected CAP Approach.

Area of Science:

• Quantum Chemistry
• Theoretical Chemistry
• Computational Chemistry

Background:

• Complex absorbing potential (CAP) is a key method for characterizing electronic resonances in quantum mechanics.
• Current CAP methods lack efficient tools for exploring resonance potential energy surfaces, such as geometry optimization.
• Analytic nuclear gradients are limited to specific electronic structure methods for CAP-based calculations.

Purpose of the Study:

• To develop a general approach for calculating nuclear gradients and nonadiabatic couplings for electronic resonances.
• To extend the applicability of CAP methods to geometry optimizations and dynamical simulations of resonance states.
• To enable the use of various electronic structure methods for resonance characterization.

Main Methods:

• Utilized the projected complex absorbing potential (CAP) technique.
• Extended bound-state gradients and nonadiabatic couplings to resonance states.
• Applied the method to state-averaged complete active space self-consistent field (SA-CASSCF) and multireference configuration interaction with single excitations (MRCI) methods.

Main Results:

• Developed a general approach for analytic nuclear gradients and nonadiabatic couplings for resonances.
• Successfully applied the method to SA-CASSCF and MRCI electronic structure theories.
• Reported accurate equilibrium geometries for temporary anions like N2-, H2CO-, HCOOH-, and C2H4-.

Conclusions:

• The projected CAP technique provides a versatile framework for calculating resonance properties.
• The developed method significantly enhances the capability for studying resonance phenomena in molecular systems.
• This work paves the way for more detailed investigations of temporary anions and other resonance states.