Dirk V. Porezag

A Self-Consistent charge density-functional based tight-binding method for predictive materials simulations in physics, chemistry and biology

We outline recent developments in quantum mechanical atomistic modelling of complex materials properties that combine the efficiency of semi-empirical quantum-chemistry and tight-binding approaches with the accuracy and transferability of more sophisticated density-functional and post-Hartree-Fock methods with the aim to perform highly predictive materials simulations of technological relevant sizes in physics, chemistry and biology. Following Harris, Foulkes and Haydock, the methods are based on an expansion of the Kohn-Sham total energy in density-functional theory (DFT) with respect to charge density fluctuations at a given reference density. While the zeroth order approach is equivalent to a common standard non-self-consistent tight-binding (TB) scheme, at second order by variationally treating the approximate Kohn-Sham energy a transparent, parameter-free, and readily calculable expression for generalized Hamiltonian matrix elements may be derived. These matrix elements are modified by a Self-Consistent redistribution of Mulliken Charges (SCC). Besides the usual “band-structure” and short-range repulsive terms the final approximate Kohn-Sham energy explicitly includes Coulomb interaction between charge fluctuations. The new SCC-scheme is shown to successfully apply to problems, where defficiencies within the non-SCC standard TB-approach become obvious. These cover defect calculations and surface studies in polar semiconductors (see M. Haugk et al. of this special issue), spectroscopic studies of organic light-emitting thin films, briefly outlined in the present article, and atomistic investigations of biomolecules (see M. Elstner et al. of this special issue).

Strategies for Massively Parallel Local‐Orbital‐Based Electronic Structure Methods

We discuss several aspects related to massively parallel electronic structure calculations using the gaussian-orbital based Naval Research Laboratory Molecular Orbital Library (NRLMOL). While much of the discussion is specific to gaussian-orbital methods, we show that all of the computationally intensive problems encountered in this code are special cases of a general class of problems which allow for the generation of parallel code that is automatically dynamically load balanced. We refer to the algorithms for parallelizing such problems as “honey-bee algorithms” because they are analogous to natures way of generating honey. With the use of such algorithms, BEOWULF clusters of personal computers are roughly equivalent to higher performance systems on a per processor basis. Further, we show that these algorithms are compatible with more complicated parallel programming architectures that are reasonable to anticipate. After specifically discussing several parallel algorithms, we discuss applications of this program to magnetic molecules.

Density functional based studies of transition states and barriers for hydrogen exchange and abstraction reactions

The overbinding that is inherent in existing local approximations to the density functional formalism has limited the usefulness of the local density approximation (LDA) for describing phenomena that are mediated by reaction barriers. Since the generalized gradient approximation (GGA) significantly decreases the overbinding, prospects for density functional based reaction dynamics are promising. Using both LDA and GGA functionals, we determined the transition state properties for four different reactions; H2+H→H+H2, CH4+H→CH3+H2,H+CH4→CH4 +H, and CH4+CH3 →CH3+CH4. Although we find that GGA still underestimates reaction barriers, our results show that this functional leads to significant improvements of the calculated reaction barriers and energetics.

Vibrational signatures of fullerene oxides

Raman and IR spectra of C60, C60O, C120O and C120O2, measured at room temperature, are discussed using geometric structures, normal modes, Raman and IR intensities from density functional-based tight-binding (DF-TB) calculations. An assignment of the most important vibrational modes is given. In the studied compounds the splitting of the C60 modes is explained by the removal of degeneracy and the vibrational coupling between the C60 cages. Strong IR absorptions of both dimeric oxides between 950 and 1150 cm-1 are assigned to the furan-like structures connecting the buckyballs. The inter-ball Raman lines below 200 cm-1 are shown to be dependent on the bonding strength between the cages. Both the bridge-based IR absorptions and the inter-ball Raman modes are suitable for the identification of the nature and number of bridging structures in oligomeric fullerene oxides.

Structural and vibrational properties of C60 oligomers

Abstract We have applied a density-functional based nonorthogonal tight-binding (DF–TB) method to study the structure, energetics and vibrational properties of different [C60]n oligomers, n=2,3,4. We determine the stable equilibrium configurations and present Raman intensities for their vibrations within a bond polarization model, which allow the identification of various oligomer species in comparison with experiments.

Theoretical calculations of magnetic order and anisotropy energies in molecular magnets

We present theoretical electronic structure calculations on the nature of electronic states and the magnetic coupling in the Mn12O12 free cluster and the Mn12O12(RCOO)16(H2O)4 molecular magnetic crystal. The calculations have been performed with the all-electron full-potential NRLMOL code. We find that the free Mn12O12 cluster relaxes to an antiferromagnetic cluster with no net moment. However, when coordinated by sixteen HCOO ligands and four H2O groups, as it is in the molecular crystal, we find that the ferrimagnetic ordering and geometrical and magnetic structure observed in the experiments is restored. Local Mn moments for the free and ligandated molecular magnets are presented and compared to experiment. We identify the occupied and unoccupied electronic states that are most responsible for the formation of the large anisotropy barrier and use a recently developed full-space and full-potential method for calculating the spin–orbit coupling interaction and anisotropy energies. Our calculated second-o...

Partial Pressure of Phosphorus and Arsenic Vapor Measured by Raman Scattering

In-situ Raman scattering results on vapor of phosphorus and arsenic at temperatures up to 1400 K are presented. The ratio of the Raman intensities of different species in the gas is proportional to the ratio of the corresponding partial pressures. The results of this new method for measuring partial pressures will be compared with thermodynamical calculations. The presented method allows the on line monitoring of the vapor atmosphere during crystal growth or annealing processes, which may be important for optimizing growth conditions, doping by the gas phase or quality optimizing of wafers.

The composition of phosphorus and arsenic vapor in view of thermal processing of III-V wafers

In-situ Raman scattering results on vapor of phosphorus and arsenic at temperatures up to 1400 K are presented. The ratio of partial pressures of the dimer and the tetramer is proportional to the ratio of the corresponding Raman intensities. With respect to the quantitative comparison of the experimental Raman data with thermodynamical calculation (ChernSage) the Raman activities were determined by first principles calculations on these molecules. In particular, we comment on the presence of trimer components in these gases as conjectured in thermodynamics.

Molecular-Dynamic Simulations of Structure Formation in Complex Materials

We are describing fundamental principles for molecular-dynamic simulations of structure formation in real materials at finite temperature. Various concepts for the calculation of total energies and interatomic forces are reviewed: Classical concepts based on the construction of empirical potentials and quantum-mechanical concepts combining the atom dynamics with a simultaneous solution of the electron problem of the many-atom configuration within density-functional theory. Out of these concepts we introduce in more detail a parameter-free density-functional-based nonorthogonal tight-binding scheme. This method combines the advantages of the simplicity and efficiency of semiempirical tight-binding approaches with the accuracy and transferability of ab initio calculations. After describing the simulation geometries and regimes for clusters, bulk structures and surface modifications the accuracy and high transferability of the interatomic potentials to the simulations of all-scale systems including also heteronuclear interactions are verified. Various successful applications of the method to the study of C60-polymerization, the stability of highly tetrahedral amorphous carbon and the characterization of diamond surface reconstructions are summarized.

Electronic and vibrational spectroscopy of fullerene-based materials

Recent work on the electronic and vibrational spectra of fullerenes and fullerene assembled materials is discussed. To aid in identifying the potentially useful polymeric phase of fullerene-based materials we compare various aspects of the spectra of the isolated and dimerized fullerene molecules. We discuss the core-level shifts of the spectra that are induced by polymerization and the changes in the electronic density of states near the Fermi level. We discuss three qualitative changes that are expected to occur in the vibrational spectrum upon polymerization and present the calculated infrared (IR) and Raman vibrational spectra for both the isolated and dimerized fullerenes. Some of the energetics associated with dimerization are also briefly discussed.