Communication: Analytic gradients for the complex absorbing potential equation-of-motion coupled-cluster method

Area of Science:

• Quantum Chemistry
• Computational Chemistry
• Theoretical Chemistry

Background:

• Equation-of-motion coupled-cluster (EOM-CC) methods are vital for electronic structure calculations.
• Calculating properties of resonance states, such as temporary anions, presents unique challenges.
• Complex Absorbing Potential (CAP) methods offer a way to study these short-lived species.

Purpose of the Study:

• To present the general theory of analytic energy gradients for the CAP-EOM-CC method.
• To implement and validate these gradients for the singles and doubles approximation.
• To enable geometry optimizations of resonance states in polyatomic molecules.

Main Methods:

• Derivation of CAP-EOM-CC energy gradients using a Lagrangian formalism.
• Focus on additional terms introduced by the Complex Absorbing Potential (CAP).
• Implementation within the singles and doubles (SD) approximation.

Main Results:

• Successful implementation of analytic energy gradients for CAP-EOM-CC.
• Demonstrated ability to locate minima on complex-valued potential energy surfaces.
• Computed equilibrium structures and adiabatic electron affinities for temporary anions of formaldehyde, formic acid, and ethylene.

Conclusions:

• The developed CAP-EOM-CC gradients facilitate geometry optimizations of resonance states.
• This method provides an advantage over standard EOM-CC for studying temporary anions.
• The approach is applicable to high-dimensional potential energy surfaces of polyatomic molecules.