An Efficient Algorithm for Constrained CASSCF(1,2) and CASSCF(3,2) Simulations as Relevant to Electron and Hole...

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Theoretical Chemistry

Background:

• Studying charge transfer states and D1-D0 crossings is crucial for understanding photochemical and photophysical processes.
• Existing methods for these studies, particularly for systems with odd numbers of electrons, can be computationally expensive.
• The electron/hole-transfer Dynamical-weighted State-averaged Constrained CASSCF (eDSC/hDSC) method was recently developed for these investigations.

Purpose of the Study:

• To propose an efficient computational algorithm for the eDSC/hDSC method.
• To reduce the computational cost associated with studying charge transfer states and D1-D0 crossings in systems with odd numbers of electrons.
• To enable faster nonadiabatic dynamics simulations in the future.

Main Methods:

• Developed a novel algorithm by separating the constrained minimization into unconstrained self-consistent-field (SCF) and constrained nonself-consistent-field (nSCF) problems.
• Accelerated the SCF problem solution using an optimized direct inversion in the iterative subspace (DIIS) technique.
• Compared the computational cost against the standard sequential quadratic programming (SQP) approach.

Main Results:

• The proposed algorithm reduces the computational cost by a factor of 8-20 compared to direct SQP methods.
• The method effectively handles charge transfer states and D1-D0 crossings for systems with odd numbers of electrons.
• The algorithm demonstrates significant computational efficiency gains.

Conclusions:

• The developed efficient algorithm significantly lowers the computational burden of the eDSC/hDSC method.
• This approach offers a more practical way to study complex electronic processes like charge transfer and conical intersections.
• The methodology is expected to be extendable to other constrained minimization problems and pave the way for rapid nonadiabatic dynamics simulations.