DFTB/PCM Applied to Ground and Excited State Potential Energy Surfaces

Accounting for solvent effects in quantum chemical calculations is vital for the accurate description of potential energy surfaces in solution. In this study, we derive a formulation of the analytical first-order geometrical derivative of ground- and excited-state energies within the time-dependent density-functional tight-binding (TD-DFTB) method with the polarizable continuum model (PCM), TD-DFTB/PCM. The performance of this is then evaluated for a series of halogen-exchange S(N)2 reactions. DFTB/PCM reproduces DFT results well for isolated monohalogenated methanes, but its agreement for transition structures significantly depends on the halogen element. The performance of TD-DFTB/PCM is evaluated for the excited state intramolecular proton transfer (ESIPT) reaction of 3-hydroxyflavone (3HF) in ethanol. TD-DFTB/PCM reproduces the barrier height of the ESIPT reaction in terms of geometry and energy relatively well, but it fails to reproduce the experimental absorption and fluorescence energies as a consequence of the absence of long-range corrections. Computational timings with and without PCM show that the additional cost of PCM for C500H502 is only 10% greater than the corresponding calculation in vacuum. Furthermore, the potential applications of TD-DFTB/PCM are highlighted by applying it to a double-stranded DNA complexed with dye (PDB ID 108D). We conclude that TD-DFTB/PCM single-point calculations and geometry optimizations for systems consisting of more than 1000 and 500 atoms, respectively, is now manageable and that properties predicted with TD-DFTB must be interpreted with care.