Automated Active Space Selection with CASCI Dipole Moments

Area of Science:

• Quantum Chemistry
• Computational Chemistry
• Theoretical Chemistry

Background:

• Multireference calculations are crucial for highly correlated systems.
• Accurate active space selection is vital for the efficiency and success of these methods.
• Manual active space selection is prone to human error and limits high-throughput applications.

Purpose of the Study:

• To develop and validate enhanced automated active space selection protocols.
• To improve the efficiency of active space selection while maintaining accuracy in excitation energy calculations.
• To extend the applicability of these protocols to a wider range of molecular systems and properties.

Main Methods:

• Utilizing complete active space configuration interaction (CASCI) dipole moments with second-order Møller-Plesset perturbation theory orbitals within dipole moment active space selection (DM-AS) protocols.
• Introducing new protocols, CASCI-D²DM-AS and its directional variant, specifically designed for charge transfer states.
• Applying the developed protocols to calculate excitation energies, ground-state spin states, and bond dissociation curves for various molecular systems.

Main Results:

• The new DM-AS protocols using CASCI dipole moments significantly improve selection efficiency compared to previous CASSCF-based methods.
• The efficiency gains are achieved while maintaining reasonable accuracy for complete active space second-order perturbation theory (CASPT2) and complete active space pair-density functional theory (CAS-PDFT) vertical excitation energies.
• The protocols demonstrate broad applicability across small organic molecules, larger conjugated systems, transition metal oxides, and bimolecular complexes.

Conclusions:

• The refined automated active space selection protocols offer a more efficient and robust approach for multireference calculations.
• These methods enhance the predictive power of computational chemistry for complex molecular systems and properties.
• The developed protocols are valuable tools for high-throughput computational chemistry and accurate electronic structure investigations.