# Fast Methods For Multisite Charge Transfer Processes I: Constrained, State Averaged CASSCF(1,M) and CASSCF(2M-1,M) Simulations

## TL;DR

This paper introduces a new constrained, state-averaged CASSCF method tailored for simulating charge transfer across multiple molecular fragments, enabling efficient nonadiabatic dynamics simulations.

## Contribution

It develops a dynamically-weighted, constrained CASSCF approach for multi-fragment charge transfer, improving computational efficiency and applicability to complex systems.

## Key findings

- Successfully modeled exponential decay of diabatic couplings in SSH chain
- Efficiently solved constrained optimization with DIIS-SQP algorithm
- Enabled nonadiabatic dynamics simulations for multi-state charge transfer

## Abstract

We design a dynamically-weighted state-averaged constrained CASSCF to treat \ul{e}lectrons or \ul{h}oles moving between $n$ molecular fragments (where $n$ can be larger than 2). Within such a so-called eDSCn/hDSCn approach, we consider configurations that are mutually single excitations of each other, and we apply a generalized set of constraints to tailor the method for studying charge transfer problems. The constrained optimization problem is efficiently solved using a DIIS-SQP algorithm, thus maintaining computational efficiency. We demonstrate the method for a finite Su-Schrieffer-Heeger (SSH) chain, successfully reproducing the expected exponential decay of diabatic couplings with distance. When combined with a gradient, the current extension immediately enables efficient nonadiabatic dynamics simulations of complex multi-state charge transfer processes.