An Efficient Algorithm for Constrained CASSCF(1,2)\nand CASSCF(3,2) Simulations as Relevant to Electron and Hole Transfer\nProblems

We\npropose an efficient algorithm for the recently published electron/hole-transfer\nDynamical-weighted State-averaged Constrained CASSCF (eDSC/hDSC) method\nstudying charge transfer states and D<sub>1</sub>-D<sub>0</sub> crossings\nfor systems with odd numbers of electrons. By separating the constrained\nminimization problem into an unconstrained self-consistent-field (SCF)\nproblem and a constrained nonself-consistent-field (nSCF) problem,\nas well as accelerating the direct inversion in the iterative subspace\n(DIIS) technique to solve the SCF problem, the overall computational\ncost is reduced by a factor of 8–20 compared with directly\nusing sequential quadratic programming (SQP). This approach should\nbe applicable for other constrained minimization problems, and in\nthe immediate future, once gradients are available, the present eDSC/hDSC\nalgorithm should allow for speedy nonadiabatic dynamics simulations.