Robin Hirschl

The Perdew–Burke–Ernzerhof exchange-correlation functional applied to the G2-1 test set using a plane-wave basis set

Present local and semilocal functionals show significant errors, for instance, in the energetics of small molecules and in the description of band gaps. One possible solution to these problems is the introduction of exact exchange and hybrid functionals. A plane-wave-based algorithm was implemented in VASP (Vienna ab-initio simulation package) to allow for the calculation of the exact exchange. To systematically assess the precision of the present implementation, calculations for the 55 molecules of the G2-1 quantum chemical test set were performed applying the PBE and PBE0 functionals. Excellent agreement for both atomization energies and geometries compared with the results obtained by GAUSSIAN 03 calculations using large basis sets (augmented correlation consistent polarized valence quadruple zeta for the geometry optimization and augmented correlation-consistent polarized valence quintuple zeta for the energy calculations) was found. The mean absolute error for atomization energies between VASP and the experiment is 8.6 and 3.7 kcalmol, as calculated with the PBE and PBE0 functionals, respectively. The mean deviations between VASP and GAUSSIAN are 0.46 and 0.49 kcalmol for the PBE and PBE0 functionals, respectively.

DFT study of adsorption and dissociation of thiophene molecules on Ni(110)

The different adsorption possibilities of thiophene (C4H4S) on the Ni(1 1 0) surface have been studied using first principle local-density-functional calculations, with the Vienna ab initio simulation package, which is based on a plane wave basis set and projector augmented wave potentials. For each configuration, a geometric optimisation has been performed. A detailed analysis of the structural and electronic properties of the molecule and the surface in the most stable conformations is presented, showing the combined roles of the molecular distortion and the interactions between the molecule and the surface. Three structures with comparatively large adsorption energies are identified, all with the molecule plane parallel to the surface. Starting from these stabilised structures, various scenarios for the desulfurisation process have been envisaged. While, for the most stable structure, the formation of an adsorbed thiol is an activated process, with an energetic barrier of 0.70 eV, the two structures which are just a bit less stable can dissociate to a C4H4 species and a sulfur atom with barriers as low as 0.07 eV. A description of the different transition states and a kinetic analysis of the desulfurisation reaction is also presented.

The phase diagram and electronic structure of Pd-V alloys:ab initio density functional calculations

A parameter-free approach based on ab initio density functional calculations is shown to describe the phase stability and order-disorder transformations in Pd-V substitutional alloys and intermetallic compounds with remarkable accuracy, allowing first-principles calculations of the complete alloy phase diagram. The investigations are based on electronic structure and total-energy calculations for ordered compounds and disordered alloys (treated in a supercell approximation) using gradient-corrected exchange-correlation functionals and a plane-wave-based all-electron method. All calculations involve a complete optimization of all structural degrees of freedom. The calculation of the free energies of the competing phases is based on rather simple mean-field descriptions of long- and short-range-ordering phenomena, using concentration-dependent interchange and shell interaction parameters. In addition, the electronic structures of ordered compounds and of substitutional alloys have been analysed.

Multifractal Phase-Space Distributions for Stationary Nonequilibrium Systems

The phase-space density of stationary nonequilibrium particle systems is known to be a multifractal object with an information dimension smaller than the phase-space dimension. The rate of heat flowing through the system, divided by the Boltzmann constant and the kinetic temperature, is equal to the sum of the Lyapunov exponents. The reduction in dimensionality is determined from the spectrum of Lyapunov exponents. We show here that also many-body systems in nonequilibrium states with stochastic thermostats can be found that have similar properties and support fractal structures in phase space. We study two two-dimensional examples: first, color conductivity for a system of hard disks, which are thermostated by a stochastic map which affects the momenta of randomly chosen particles; second, color conductivity of a system of soft disks which are subjected to a stochastic force and perform Brownian motion. Full Lyapunov spectra were computed for both models, and the information dimensions of their underlying attractors determined.