Molecular response properties from a Hermitian eigenvalue equation for a time-periodic Hamiltonian

Area of Science:

• Quantum chemistry
• Theoretical chemistry
• Computational chemistry

Background:

• Calculating molecular properties under time-periodic perturbations is complex.
• Existing response function theories have limitations and inconsistencies.

Purpose of the Study:

• To develop a rigorous and transparent formulation of response function theory for time-periodic perturbations.
• To unify the treatment of static and time-periodic perturbations in calculating molecular properties.
• To extend the applicability of response functions to a wider range of wave function models.

Main Methods:

• Recasting the time-dependent Schrödinger equation into a Hermitian eigenvalue equation.
• Defining molecular properties as derivatives of quasi-energy with respect to perturbation strengths.
• Utilizing variational principles or projection methods to determine quasi-energy.
• Introducing the coupled cluster configuration interaction (CC-CI) model.

Main Results:

• A unified framework for calculating molecular properties under static and time-periodic perturbations.
• The CC-CI model provides analytical analogues to finite field EOMCC calculations for static perturbations.
• Identified similarities and differences in response functions for CI, CC, and CC-CI models, highlighting parametrization effects.

Conclusions:

• The new formulation resolves inaccuracies in previous response function theories.
• The developed framework enhances the universality and applicability of response function theory.
• The CC-CI model offers a novel approach combining the strengths of coupled cluster and configuration interaction methods.