Analytic first-order derivatives of CASPT2 with IPEA shift

Area of Science:

• Quantum Chemistry
• Computational Chemistry
• Theoretical Chemistry

Background:

• Complete active space second-order perturbation theory (CASPT2) is a valuable tool for electronic structure calculations.
• CASPT2 is known to systematically underestimate excitation energies.
• The ionization potential-electron affinity (IPEA) shift can correct this underestimation.

Purpose of the Study:

• To develop analytic first-order derivatives of CASPT2 incorporating the IPEA shift.
• To address the invariance issue of CASPT2-IPEA with respect to active molecular orbital rotations.
• To apply the developed method for locating minimum energy structures and conical intersections.

Main Methods:

• Development of analytic first-order derivatives for CASPT2 with IPEA shift.
• Inclusion of two additional constraint conditions in the CASPT2 Lagrangian.
• Application to methylpyrimidine derivatives and cytosine.

Main Results:

• The developed CASPT2-IPEA method with analytic derivatives was successfully applied.
• Minimum energy structures and conical intersections were located for the studied molecules.
• Inclusion of the IPEA shift improved the agreement of calculated energies with experimental and high-level data.

Conclusions:

• Analytic first-order derivatives of CASPT2 with the IPEA shift provide improved accuracy for electronic structure properties.
• The method enhances the prediction of excitation energies and molecular geometries.
• This advancement is beneficial for studying complex electronic systems like methylpyrimidines and cytosine.