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.

The vibrational modes of H 2 molecules in Si are found using a first-principles method and compared with recent experimental investigations. The isolated molecule is found to lie at a T d interstitial site, oriented along ͓011͔ and is infrared active. The rotational barrier is at least 0.17 eV. The molecular frequency is a sensitive function of cage size and increases to lie close to the gas value for cages about 50% larger than the T d site. It is suggested that Raman-active modes around 4158 cm Ϫ1 are due to molecules within voids.

We report benchmark calculations of the density functional based tight-binding method concerning the magnetic properties of small iron clusters (Fe2 to Fe5) and the Fe13 icosahedron. Energetics and stability with respect to changes of cluster geometry of collinear and noncollinear spin configurations are in good agreement with ab initio results. The inclusion of spin-orbit coupling has been tested for the iron dimer.