The density functional based tight binding method (DFTB) is an approximation of Kohn-Sham (KS) DFT wherein the energy functional is expanded to second order with respect to density fluctuations. Application of a ΔSCF approach within DFTB has allowed for the variationally optimized calculation of spin-purified excited state properties. Selection of KS spin orbitals based on the character of the excited state, and subsequent relaxation of these orbitals under non-Aufbau occupation constraints for both the singlet and triplet configuration is followed by application of the Ziegler sum rule to determine the spin purified excited state of the system, its energy, and its optimized geometry. While spectroscopic information can be derived from ΔDFTB excited state molecular dynamics trajectories, direct characterization of the intensity of spectral peaks requires evaluation of the electronic transition dipole moment (TDM). The TDM is also useful in deriving rates of energy transfer in excited donor-acceptor systems. Calculation of TDMs within a DFTB framework is complicated by the fact that the variationally optimized orbitals for the ground state are not mutually orthogonal with those of the excited state . Therefore a corresponding orbital transformation must be applied to the molecular orbitals of the ground and excited-state to ensure orthogonality. Benchmarks of permanent and transition dipole moments from excited-state DFTB will be directly compared to those obtained through EOM-CCSD and TD-DFT. The implementation of TDMs within a time-independent DFTB framework creates an accessible method for high throughput screening of large photoactive systems with application in alternative energy solutions.

Evaluation of electronic dipole moments in time-independent excited-state DFTB

2019 Spring National Meeting

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