Analytical gradients and derivative couplings for dynamically weighted complete active space self-consistent field.

Area of Science:

• Quantum chemistry
• Theoretical chemistry
• Computational chemistry

Background:

• Excited-state dynamics are crucial for understanding photochemical processes.
• Previous methods for calculating excited-state dynamics were computationally intensive.
• The Dynamically Weighted Complete Active Space Self-Consistent Field (DW-CASSCF) method was recently introduced.

Purpose of the Study:

• To reformulate analytical gradients for the DW-CASSCF method using a Lagrangian approach.
• To derive and implement derivative couplings at the DW-CASSCF level for the first time.
• To apply the new formulation to optimize a conical intersection for the p-hydroxybenzylidene-imidazolinone anion.

Main Methods:

• Lagrangian approach for analytical gradients.
• Reduced coupled-perturbed CASSCF calculations to one per state gradient.
• Implementation of derivative couplings at the DW-CASSCF level.

Main Results:

• Efficient reformulation of DW-CASSCF analytical gradients.
• First-time derivation and implementation of derivative couplings for DW-CASSCF.
• Successful optimization of a conical intersection for the green fluorescent protein chromophore.

Conclusions:

• The new DW-CASSCF gradient formulation significantly enhances computational efficiency.
• The inclusion of derivative couplings opens new avenues for studying excited-state dynamics.
• This work provides insights into the radiationless decay mechanisms of the green fluorescent protein chromophore.