Active orbital preservation for multiconfigurational self-consistent field.

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Spectroscopy

Background:

• Accurate electronic structure calculations are crucial for understanding molecular properties.
• Maintaining orbital consistency across molecular configurations is challenging in quantum chemistry.
• Multiconfigurational Self-Consistent Field (MCSCF) methods require stable active spaces for reliable results.

Purpose of the Study:

• To introduce an automated method, Active Orbital Preservation for Multiconfigurational Self-Consistent Field (AOP-MCSCF), for preserving active space orbital consistency.
• To enable accurate electronic structure calculations for molecules across various configurations, including solvated systems.
• To validate the AOP-MCSCF approach on a relevant chemical system and compare results with experimental data.

Main Methods:

• Developed AOP-MCSCF utilizing maximum overlap with reference orbitals.
• Employed singular value decomposition of a Molecular Orbital (MO) overlap matrix for orbital rotation.
• Used rotated orbitals as initial guesses for MCSCF calculations.
• Applied the method to aqueous p-hydroxybenzylidene-imidazolinone (HBI) with quantum-mechanically treated water.

Main Results:

• AOP-MCSCF successfully converged to correct orbitals for over 90% of 3000 thermally sampled configurations of aqueous HBI.
• Computed linear absorption spectrum showed excellent agreement with experimental measurements up to 5.4 eV (230 nm).
• Electrostatic contributions to solvation energy were identified as key drivers of state-dependent solvatochromism in HBI.

Conclusions:

• AOP-MCSCF provides a robust automated solution for maintaining orbital consistency in complex molecular systems.
• The method significantly enhances the reliability of MCSCF calculations for dynamic and solvated environments.
• Accurate theoretical predictions of spectroscopic properties are achievable, aiding in the interpretation of experimental data.