# Fast Methods For Multisite Charge Transfer Processes II: Analytic Nuclear Gradients and Nonadiabatic Dynamics For cCASSCF(1,M) and cCASSCF(2M-1,M) Wavefunctions

## TL;DR

This paper develops and implements analytic nuclear gradients and derivative couplings for a specialized CASSCF method, enabling efficient nonadiabatic dynamics simulations of charge transfer processes.

## Contribution

It introduces a new analytic gradient and coupling approach for constrained CASSCF(1,M) and (2M-1,M) wavefunctions, facilitating charge transfer modeling.

## Key findings

- Enabled initial surface-hopping simulations of proton-coupled electron transfer.
- Provided a computational framework for smooth potential energy surfaces.
- Demonstrated applicability to phenoxyl-phenol system.

## Abstract

We derive and implement analytic nuclear gradients and derivative couplings for a constrained Complete Active Space Self-Consistent Field with a small active space designed to model electron or hole transfer. Using a Lagrangian formalism, we are able to differentiate both the CASSCF energy and the constraint (which is required for globally smooth surfaces), and the resulting efficient algorithm can be immediately applied to nonadiabatic dynamics simulations of charge transfer processes. Here, we run initial surface-hopping simulations of a proton coupled electron transfer event for a phenoxyl-phenol system.