DFTB3 is a recent extension of the self-consistent-charge density-functional tight-binding method (SCC-DFTB) and derived from a third order expansion of the density functional theory (DFT) total energy around a given reference density. Being applied in combination with the parametrization of its predecessor (MIO), DFTB3 improves for hydrogen binding energies, proton affinities, and hydrogen transfer barriers. In the present study, parameters especially designed for DFTB3 are presented, and its performance is evaluated for small organic molecules focusing on thermochemistry, geometries, and vibrational frequencies from our own and several databases from literature. The new parameters remove significant overbinding errors, reduce errors for geometries of noncovalent interactions, and improve the overall performance.

No abstract

No abstract

Due to steadily rising computational power and sophisticated modeling approaches, increasingly larger molecular systems can be modeled with ab initio methods. An especially promising approach is subsystem methods, where the total system is broken down into smaller subunits that can be treated individually. If an implicit solvent environment such as the conductor-like screening model (COSMO) is included in the description, then additional environmental effects can be incorporated at relatively low cost. For very large systems described with subsystem methods, however, the solution of the COSMO equations can actually become the bottleneck of the calculation. A prominent example in this area is biomolecular systems such as proteins, which can, for instance, be described with frozen density embedding (FDE), especially the related 3-FDE approach. In this article, we present an alternative COSMO variant, which exploits the subsystem nature of the underlying electronic description and has been implemented in a development version of the Amsterdam Density Functional program suite (Adf). We show that the computational cost for the solvent model can be reduced dramatically while retaining the accuracy of the regular description. We compare several schemes for density and surface charge updates and assess the effect of the single tuning parameter.

The Three-Partition Frozen Density Embedding (3-FDE) method is applied to the photosynthetic Fenna-Matthews-Olson (FMO) complex of green sulphur bacteria in order to determine cofactor excitation energies. We present a sophisticated multi-layer embedding protocol, which allows to choose fragments of arbitrary size. This is useful in order to limit the total number of calculations, which can create a significant overhead. The approach allows to determine fully polarised densities for realistic proteins in this framework for the first time. A solvent shell around the total system is described in terms of the LoCOSMO algorithm, which tremendously simplifies the computational effort of a continuum description around a system of this size. The solvent response to the excitation is incorporated by a new extension of the LoCOSMO scheme to time-dependent density functional theory. Excitation energies for the chromophores are calculated under the influence of density-based embedding potentials obtained with different technical settings. It is found that protein density relaxation in this particular example hardly influences the transitions, indicating that embedding of the pigments in the initial protein density is sufficient to determine site energy modulations with our approach.