Combination of a Voronoi-Type Complex Absorbing Potential with the XMS-CASPT2 Method and Pilot Applications.

Area of Science:

• Quantum Chemistry
• Computational Physics
• Theoretical Chemistry

Background:

• Electronic resonances are challenging (N+1)-electron metastable states embedded in a continuum.
• Accurate description requires balancing bound and continuum state aspects.
• Existing methods face challenges in describing resonance decay.

Purpose of the Study:

• To develop and apply a computational method for accurately describing electronic resonances.
• To address the challenge of balancing bound and continuum aspects in resonance calculations.
• To benchmark the new method against existing techniques and explore its capabilities.

Main Methods:

• A smoothed Voronoi complex absorbing potential (CAP) was combined with the XMS-CASPT2 method.
• The method was implemented in a projected scheme to reduce computational cost.
• Systematic examination of convergence with respect to active space size was performed.

Main Results:

• The CAP/XMS-CASPT2 method accurately predicts resonance parameters for formaldehyde, chloroethene, and N2- anions.
• Results show good agreement with CAP/SAC-CI methods (approx. 0.15 eV deviation).
• The method demonstrates clear advantages in bond dissociation regions and for multireference states.

Conclusions:

• The CAP/XMS-CASPT2 method provides a balanced description of electronic resonances, overcoming key challenges.
• It offers advantages over existing methods, particularly in bond dissociation scenarios.
• The approach is efficient and capable of describing complex multireference states with modest active spaces.