Implementation of a Constraint and Configuration Interaction Methodology Into Density Functional Tight Binding

This research aims to implement a charge constraint in conjunction with a small configuration interaction scheme into a density-functional tight-binding (DFTB) method within the DFTB+ quantum mechanical software package. This method aims to model the electron transfer rate of chemical systems by calculating the electronic couplings between two constrained states more efficiently. Electronic couplings are directly proportional to electron transfer, making them important parameters to efficiently compute the optimal minimum or maximum of an electron transfer rate, for example, when screening chemical systems based on their ability as a conductor. Other methods such as constrained density-functional theory followed by a small configuration interaction scheme (CDFT-CI) developed by Wu and Van Voorhis can calculate electronic couplings. Still, as the complexity of chemical systems increases, the computational cost of CDFT-CI becomes intractable. Using CDFT-CI as a starting point, we can develop a constrained density-functional tight-binding followed by a small configuration interaction scheme (CDFTB-CI) to lower computational costs compared to CDFT-CI. The strategies to implement a CDFTB-CI option into DFTB+ utilize built-in features of DFTB+ while being as non-intrusive as possible. This process introduces a constraint option in DFTB+ with the capabilities of calculating constrained energies of constrained states of simple molecules, such as a set of simple homogeneous and heterogeneous dimers. This set of simple molecules can be used as case studies with the implications of finding the best practices for CDFTB.