Fully relativistic complete active space self-consistent field for large molecules: quasi-second-order minimax...

Area of Science:

• Quantum Chemistry
• Relativistic Effects
• Computational Chemistry

Background:

• Accurate treatment of relativistic effects is crucial for heavy elements.
• Complete Active Space Self-Consistent Field (CASSCF) methods are computationally demanding.
• Existing methods struggle with large systems containing heavy elements.

Purpose of the Study:

• To develop an efficient algorithm for four-component CASSCF methods.
• To incorporate spin-orbit and other relativistic effects self-consistently.
• To enable routine calculations on large molecular systems with heavy elements.

Main Methods:

• Developed an efficient algorithm based on the Dirac equation.
• Employed a trust-region quasi-Newton method for orbital optimization.
• Utilized density fitting and parallel computation for efficiency.

Main Results:

• Demonstrated routine Dirac-Coulomb CASSCF calculations for systems up to 100 atoms with heavy elements.
• Presented convergence behavior and wall times for specific molecular complexes.
• Reported excitation energies for octachloridodirhenate(III) using a state-averaged variant.

Conclusions:

• The developed algorithm significantly enhances the efficiency of relativistic quantum chemical calculations.
• This method facilitates the study of complex systems involving heavy elements.
• The approach is suitable for routine calculations and accurate prediction of molecular properties.