A new Fortran 95 implementation of the DFTB (density functional-based tight binding) method has been developed, where the sparsity of the DFTB system of equations has been exploited. Conventional dense algebra is used only to evaluate the eigenproblems of the system and long-range Coulombic terms, but drop-in O(N) or O(N2) modules are planned to replace the small code sections that these entail. The developed sparse storage structure is discussed in detail, and a short overview of other features of the new code is given.

DFTB+ is a versatile community developed open source software package offering fast and efficient methods for carrying out atomistic quantum mechanical simulations. By implementing various methods approximating density functional theory (DFT), such as the density functional based tight binding (DFTB) and the extended tight binding method, it enables simulations of large systems and long timescales with reasonable accuracy while being considerably faster for typical simulations than the respective ab initio methods. Based on the DFTB framework, it additionally offers approximated versions of various DFT extensions including hybrid functionals, time dependent formalism for treating excited systems, electron transport using non-equilibrium Green’s functions, and many more. DFTB+ can be used as a user-friendly standalone application in addition to being embedded into other software packages as a library or acting as a calculation-server accessed by socket communication. We give an overview of the recently developed capabilities of the DFTB+ code, demonstrating with a few use case examples, discuss the strengths and weaknesses of the various features, and also discuss on-going developments and possible future perspectives.

A new self-consistent-charge density-functional tight-binding (SCC-DFTB) set of parameters for Ti-X pairs of elements (X = Ti, H, C, N, O, S) has been developed. The performance of this set has been tested with respect to TiO2 bulk phases and small molecular systems. It has been found that the band structures, geometric parameters, and cohesive energies of rutile and anatase polymorphs are in good agreement with the reference DFT data and with experiment. Low-index rutile and anatase surfaces were also tested. For molecular systems, binding and atomization energies close to their DFT analogues have been achieved. Large errors, however, have been found for systems in high-spin states and/or having multireference character of their wave functions. The correct performance of SCC-DFTB for surface reactions has been demonstrated via the water splitting on anatase (001) surface. The current SCC-DFTB set is a suitable tool for future in-depth investigation of chemical processes occurring on the surfaces of TiO2 polymorphs as well as for other processes of physicochemical interest.

The self-consistent charge density functional tight binding (SCC-DFTB) method is a relatively new approximate electronic structure method that is increasingly used to study biologically relevant systems in aqueous environments. There have been several gas phase cluster calculations that indicate, in some instances, an ability to predict geometries, energies, and vibrational frequencies in reasonable agreement with high level ab initio calculations. However, to date, there has been little validation of the method for bulk water properties, and no validation for the properties of the hydrated excess proton in water. Presented here is a detailed SCC-DFTB analysis of the latter two systems. This work focuses on the ability of the original SCC-DFTB method, and a modified version that includes a hydrogen bonding damping function (HBD-SCC-DFTB), to describe the structural, energetic, and dynamical nature of these aqueous systems. The SCC-DFTB and HBD-SCC-DFTB results are compared to experimental data and Car-Parrinello molecular dynamics (CPMD) simulations using the HCTH/120 gradient-corrected exchange-correlation energy functional. All simulations for these systems contained 128 water molecules, plus one additional proton in the case of the excess proton system, and were carried out in a periodic simulation box with Ewald long-range electrostatics. The liquid water structure for the original SCC-DFTB is shown to poorly reproduce bulk water properties, while the HBD-SCC-DFTB somewhat more closely represents bulk water due to an improved ability to describe hydrogen bonding energies. Both SCC-DFTB methods are found to underestimate the water dimer interaction energy, resulting in a low heat of vaporization and a significantly elevated water oxygen diffusion coefficient as compared to experiment. The addition of an excess hydrated proton to the bulk water resulted in the Zundel cation (H(5)O(2)(+)) stabilized species being the stable form of the charge defect, which diffuses at a rate similar to the underlying water diffusion. These SCC-DFTB results differ significantly from other existing computational descriptions of the hydrated excess proton in water, as well as from the available experimental data.