Constrained CASSCF(2,2) and Tight-binding Approach to Study Electron Transfer between a Molecule and Metal Cluster

One of the most important and unexplored areas of quantum chemistry is electronic structure in open quantum environments, systems with fractional charges occupying molecular subspaces. To study the electron transfer to and from the bath, one needs an electronic structure method that can give smooth ground and excited potential energy surfaces while avoiding excited states corresponding to internal excitations. For these reasons, our group has developed the constrained CASSCF method. Besides that, the simulation of large open quantum systems requires embedding and approximation strategies, which sparked our idea of approximating the two-electron interaction using a tight-binding model. 

In the first part of the presentation, I will introduce the constrained CASSCF method and how it helps to study nonadiabatic systems, especially in the strong coupling region. We will focus on the motivation for constraining the orbitals, and why this idea of optimizing both charge states is superb for generating smooth and reliable potential energy surfaces and running dynamics.

In the second part of the presentation, I will elaborate on adding tight-binding calculation features. In most eletronic structure software packages, by default, all the orbitals that do SCF are calculated ab initio. To extend these calculations to open quantum systems, we need the capacity to work with two different classes of basis functions – those with and those without two-electron integrals. Moreover, the flexibility to evaluate different parts of the Hamiltonian at different levels of accuracy is crucial. This approach helps bridge ab initio and empirical model calculations.