Gotthard Seifert

CALCULATIONS OF MOLECULES, CLUSTERS, AND SOLIDS WITH A SIMPLIFIED LCAO-DFT-LDA SCHEME

A simplified LCAO-DFT-LDA scheme for calculations of structure and electronic structure of large molecules, clusters, and solids is presented. Forces on the atoms are calculated in a semiempirical way considering the electronic states. The small computational effort of this treatment allows one to perform molecular dynamics (MD) simulations of molecules and clusters up to a few hundred atoms as well as corresponding simulations of condensed systems within the Born-Oppenheimer approximation. The accuracy of the method is illustrated by the results of calculations for a series of small molecules and clusters.

Atomistic simulations of complex materials: ground-state and excited-state properties

The present status of development of the density-functional-based tightbinding (DFTB) method is reviewed. As a two-centre approach to densityfunctional theory (DFT), it combines computational efficiency with reliability and transferability. Utilizing a minimal-basis representation of Kohn–Sham eigenstates and a superposition of optimized neutral-atom potentials and related charge densities for constructing the effective many-atom potential, all integrals are calculated within DFT. Self-consistency is included at the level of Mulliken charges rather than by self-consistently iterating electronic spin densities and effective potentials. Excited-state properties are accessible within the linear response approach to time-dependent (TD) DFT. The coupling of electronic and ionic degrees of freedom further allows us to follow the non-adiabatic structure evolution via coupled electron–ion molecular dynamics in energetic particle collisions and in the presence of ultrashort intense laser pulses. We either briefly outline or give references describing examples of applications to ground-state and excited-state properties. Addressing the scaling problems in size and time generally and for biomolecular systems in particular, we describe the implementation of the parallel ‘divide-and-conquer’ order-N method with DFTB and the coupling of the DFTB approach as a quantum method with molecular mechanics force fields.

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).

Induced magnetic fields in aromatic [n]-annulenes: interpretation of NICS tensor components

The components of nucleus-independent chemical shift (NICS) tensors for Dnhn-annulenes are discussed as indexes of the aromatic character of electronic π systems. The component corresponding to the principal axis perpendicular to the ring plane, NICSzz, is found to be a good measure for the characterisation of the π system of the ring. Isotropic NICS values at ring centres contain large influences from the σ system and from all three principal components of the NICS tensor. At large distances away from the ring center, NICSzz, which is dominated by contributions from the π system, characterizes NICS well.

Designing electrical contacts to MoS2 monolayers: a computational study.

Studying the reason why single-layer molybdenum disulfide (MoS2) appears to fall short of its promising potential in flexible nanoelectronics, we find that the nature of contacts plays a more important role than the semiconductor itself. In order to understand the nature of MoS2/metal contacts, we perform ab initio density functional theory calculations for the geometry, bonding, and electronic structure of the contact region. We find that the most common contact metal (Au) is rather inefficient for electron injection into single-layer MoS2 and propose Ti as a representative example of suitable alternative electrode materials.

Designing Electrical Contacts toMoS2Monolayers: A Computational Study

Studying the reason why single-layer molybdenum disulfide (MoS2) appears to fall short of its promising potential in flexible nanoelectronics, we find that the nature of contacts plays a more important role than the semiconductor itself. In order to understand the nature of MoS2/metal contacts, we perform ab initio density functional theory calculations for the geometry, bonding, and electronic structure of the contact region. We find that the most common contact metal (Au) is rather inefficient for electron injection into single-layer MoS2 and propose Ti as a representative example of suitable alternative electrode materials.

An Efficient a Posteriori Treatment for Dispersion Interaction in Density-Functional-Based Tight Binding.

The performance of density functional theory (DFT) (VWN-LDA, PBE-GGA, and B3LYP hybrid functionals), density-functional-based tight binding (DFTB), and ab initio methods [HF, MP2, CCSD, and CCSD(T)] for the treatment of London dispersion is investigated. Although highly correlated ab initio methods are capable of describing this phenomenon, if they are used with rather large basis sets, DFT methods are found to be inadequate for the description of H2/PAH (polycyclic aromatic hydrocarbon) interactions. As an alternative approach, an a posteriori addition of a van der Waals term to DFTB is proposed. This method provides results for H2/PAH interactions in close agreement with MP2 and higher-level ab initio methods. Bulk properties of graphite also compare well with the experimental data.

BORON-NITROGEN ANALOGUES OF THE FULLERENES : ELECTRONIC AND STRUCTURAL PROPERTIES

Abstract On the basis of a systematic density functional tight-binding study of boron-nitrogen polyhedra (BN) x composed entirely of four- and six-membered rings, it is predicted that octahedron-like structures B 12 N 12 , B 16 N 16 and B 28 N 28 are “magic” (i.e. anomalously stable) clusters. The infrared spectrum of B 12 N 12 is predicted. The similarities and differences between these “inorganic fullerenes” and the carbon-based equivalents are outlined. Ahigh stability of the (BN) x clusters is found to correlate with a large HOMO-LUMO gap.

A Self‐Consistent Charge Density‐Functional Based Tight‐Binding Scheme for Large Biomolecules

(a) Department of Physics, Harvard University, Cambridge MA 02138, USA(b) Theoretische Physik, Universita¨t Paderborn, D-33098 Paderborn, Germany(c) Molekulare Biophysik, Deutsches Krebsforschungszentrum, D-69120 Heidelberg,Germany(Received August 10, 1999)A common feature of traditional tight-binding (TB) methods is the non-self-consistent solution ofthe eigenvalue problem of a Hamiltonian operator, represented in a minimal basis set. These TBschemes have been applied mostly to solid state systems, containing atoms with similar electrone-gativities. Recently self-consistent TB schemes have been developed which now allow the treat-ment of systems where a redistribution of charges, and the related detailed charge balance be-tween the atoms, become important as e.g. in biological systems. We discuss the application ofsuch a method, a self-consistent charge density-functional based TB scheme (SCC-DFTB), to bio-logical model compounds. We present recent extensions of the method: (i) The combination of thetight binding scheme with an empirical force field, that makes large scale simulations with severalthousand atoms possible. (ii) An extension which allows a quantitative description of weak-bond-ing interactions in biological systems. The latter include an improved description of hydrogenbonding achieved by extending the basis set and improved molecular stacking interactionsachieved by incorporating the dispersion contributions empirically. In applying the method, we pre-sent benchmarks for conformational energies, geometries and frequencies of small peptides andcompare with ab initio and semiempirical quantum chemistry data. These developments provide afast and reliable method, which can handle large scale quantum molecular dynamic simulations inbiological systems.

Pentagon adjacency as a determinant of fullerene stability

Optimisation of geometries of all 40 fullerene isomers of C40, using methods from molecular mechanics and tight-binding to full abinitio SCF and DFT approaches, confirms minimisation of pentagon adjacency as a major factor in relative stability. The consensus predictions of 11 out of 12 methods are that the isomer of lowest total energy is the D2 cage with the smallest possible adjacency count, and that energies rise linearly with the number of adjacencies. Quantum mechanical methods predict a slope of 80–100 kJ mol-1 per adjacency. Molecular mechanics methods are outliers, with the Tersoff potential giving a different minimum and its Brenner modification a poor correlation and much smaller penalty.