N. Troullier

Analysis of occupied and empty electronic states of C60

Abstract We calculated the electronic structure of solid C 60 using a first principles pseudopotential local-density method. We present an analysis of the theoretical results and compare them to experimental photoemission and inverse photoemission spectra of solid C 60 . The agreement between theory and experiment is excellent. We give a simple interpretation of the electronic states of C 60 based on its quasi-spherical shape.

Electronic and structural properties of TiO2 in the rutile structure

Abstract We present self-consistent calculations for the clectronic, cohesive, and structural properties of titanium dioxide in the rutile structure. The calculations were performed within the local density approximation using ab inito pseudopotentials and a plane-wave basis. We determined the lattice parameters, cohesive energy, and bulk modulus by minimizing the total energy of the solid with respect to the lattice parameters a, c , and u . We find good agreement for the structural properties, within 2% of experimentally observed values. Our cohesive energy is larger than experiment by ∼ 12% and is consistent with other local density calculations for transition metals and their compounds. The calculated electronic band structure is in good agreement with photoemission experiments.

Algorithms for predicting the structural properties of clusters

Abstract Predicting the structure of atomic clusters is one of the outstanding problems in condensed matter physics. Traditional theoretical approaches are hindered by the large number of degrees of freedom, and the lack of symmetry in these systems. Some new computational techniques for predicting the structural properties of small silicon clusters will be illustrated. The emphasis of this effort is on simulated-annealing procedures based on Langevin dynamics. Quantum forces, derived from ab initio pseudopotential calculations, are incorporated in these simulations. These forces can be efficiently calculated using higher-order finite difference methods.

Vibrational modes of silicon nanostructures

Abstract We present a method for predicting the vibrational modes of small semiconductor clusters. We employ ab initio pseudopotentials and apply a higher-order finite difference procedure to solve the Kohn-Sham equations. We predict the vibrational modes of small silicon clusters (Si n , n = 4–7) based on their ground state structures. Our calculated vibrational modes agree very well with experimental data, and with other theoretical calculations based on quantum chemistry and tight binding methods. This comparison confirms the accuracy of the finite difference procedure for calculating not only the first order derivative of the energy, but the second derivatives as well. It also validates the accuracy of pseudopotential-local density calculations for the ground state structures for Si clusters.

Electronic and structural properties of α-berlinite (AlPO4)

Abstract The electronic and structural properties of α-berlinite (AlPO4) are examined using ab initio pseudopotentials. The electronic band structure, the total and partial densities of states, and the pseudocharge valence density are presented. We find our total density of states to be in excellent agreement with X-ray photoemission spectroscopy and in good agreement with previous tight binding calculations. We also examine the charge density in order to investigate bond character.

Ytterbium monolayer diffusion barriers at Hg1−xCdxTe/Al junctions

Single layers of Yb at the Hg1−xCdxTe(110) interface prevent Al‐Te reaction and dramatically increase the Hg concentration at the interface. Synchrotron radiation photoemission studies of the interface as a function of Al deposition show a two orders of magnitude increase in the Hg/Te core intensity ratio as a result of the interlayer‐induced change in atomic interdiffusion. Calculations of thermodynamic parameters following a semiempirical alloying model suggest that other rare earths should also act as effective diffusion barriers at mercury‐cadmium‐telluride/reactive metal junctions.

Calculating large systems with plane waves: Is it a N3 or N2 scaling problem?

Abstract It is commonly asserted that in performing large scale plane wave calculations with N plane waves, and indirect diagonalization, the N 3 orthogonalization step quickly becomes the limiting factor. Using a pre-condition Lanczos subspace diagonalization algorithm we find that while the orthogonalization will eventually become the dominate cpu restriction, this is not the limiting factor. Typically the limitation resides not with the cpu, but with the memory limitations of the current computer systems. The memory needed to store the wave functions scales as N atom 2 . This scaling “limitation” is reached before the cpu time becomes the dominate factor. We illustrate this scaling with a Na vacancy calculation using up to 2000 atoms.

Molecular dynamics simulations of dense plasmas

We have performed quantum molecular dynamics simulations of hot, dense plasmas of hydrogen over a range of temperatures (0.1–5 eV) and densities (0.0625–5 g/cc). We determine the forces quantum mechanically from density functional, extended Huckel, and tight binding techniques and move the nuclei according to the classical equations of motion. We determine pair‐correlation functions, diffusion coefficients, and electrical conductivities. We find that many‐body effects predominate in this regime. We begin to obtain agreement with the OCP and Thomas‐Fermi models only at the higher temperatures and densities.

Quantum-Molecular-Dynamics Simulations of Isotopic Mixtures of Dense, Hot Hydrogen

Calculations of isotopic binary mixtures of hydrogen over a range of densities and temperatures were performed using quantum-molecular-dynamics simulations. The electronic component was treated with three different models including density functional (local density approximation), tight binding, and effective pair potentials. Quantum-mechanical effects were found to be important even at elevated temperatures. Self-diffusion coefficients for each component became comparable in magnitude with increasing density in contradiction to simple mass-scaling rules. Mutual-diffusion coefficients tended to exceed the simple concentration average of the self-diffusion coefficients, but by less than 25%.