David C. Patton

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.

Metal-coated fullerenes: electronic, geometrical and vibrational properties of C60M62 (M=Ti and V)

Recent experiments have revealed well-defined magic numbers for transition-metal-coated fullerenes C60MN (for example, M=Ti, V, Zr, and N=50, 62, 72, 80). We investigate the origin of the stability of such metal-coated fullerenes through first-principles calculations and compare these structures to other forms of clusters containing C and transition metals. Although, the transition-metal coatings disrupt the π bonding on the fullerene cage, we find that the clusters should be metastable at low temperatures or laser fluence. We discuss relative stabilities, the electronic and vibrational propertries, and the nature of bonding in the representative C60Ti62 cluster.

The Generalized-Gradient Approximation to Density Functional Theory and Bonding

A simplified version of the generalized-gradient approximation (GGA) featuring recent implementational improvements has been employed in a study of chemical bonding within the GGA and LDA. Ionic, covalent, metallic, and van der Waals bonding is discussed. Detailed calculations of bond lengths, atomization energies, and vibrational frequencies are presented for a selection of diatomic molecules. We find that the GGA performs well for a variety of systems including strongly bound dimers, weakly bound van der Waals molecules, and antiferromagnetic systems. In addition, the GGA leads to an improvement in the calculation of dimers containing hydrogen. We comment on why the GGA-induced changes of hydrogenic bonding are different than for the other atoms in the periodic table.