Communication: automatic code generation enables nuclear gradient computations for fully internally contracted...

Area of Science:

• Quantum Chemistry
• Computational Chemistry
• Theoretical Chemistry

Background:

• Accurate calculation of molecular properties is crucial for understanding chemical reactions and molecular behavior.
• Complete Active Space Second-Order Perturbation Theory (CASPT2) is a powerful method for describing electronic structures, but analytical gradients are computationally demanding.
• Efficient and accurate calculation of energy derivatives is essential for geometry optimization and reaction pathway analysis.

Purpose of the Study:

• To report the implementation of analytical nuclear gradients for fully internally contracted CASPT2.
• To develop an automated code generator for handling CASPT2 energy and its derivatives.
• To demonstrate the capability of the new implementation through calculations on the porphin molecule.

Main Methods:

• Development of an automated code generator for spin-free CASPT2 energy and derivative formulas.
• Solution of the underlying complete active space self-consistent field (CASSCF) and Z-vector equations using density fitting.
• Application to calculate vertical and adiabatic ionization potentials.

Main Results:

• Successful implementation of analytical nuclear gradients for internally contracted CASPT2.
• The automated code generator efficiently handles complex derivative calculations.
• Accurate calculation of vertical and adiabatic ionization potentials for porphin.

Conclusions:

• The developed method provides an efficient and accurate way to compute analytical nuclear gradients for CASPT2.
• This implementation facilitates geometry optimizations and reaction dynamics studies in quantum chemistry.
• The porphin ionization potential calculations validate the robustness and applicability of the new approach.