Model Hessian for accelerating first-principles structure optimizations

We present two methods to accelerate first-principles structural relaxations, both based on the dynamical matrix obtained from a universal model of springs for bond stretching and bending. Despite its simplicity, the normal modes of this model Hessian represent excellent internal coordinates for molecules and solids irrespective of coordination, capturing not only the long-wavelength acoustic modes of large systems, but also the short-wavelength low-frequency modes that appear in complex systems. In the first method, the model Hessian is used to precondition a conjugate gradients minimization, thereby drastically reducing the effective spectral width and thus obtaining a substantial improvement of convergence. The same Hessian is used in the second method as a starting point of a quasi-Newton algorithm (Broyden's method and modifications thereof), reducing the number of steps needed to find the correct Hessian. Results for both methods are presented for geometry optimizations of clusters, slabs, and biomolecules, with speed-up factors between 2 and 8.Comment: 5 pages, 3 figures submites to Phys. Rev.

Theoretical Modeling of Catalytic Processes: Hydrocarbon Oxidation and Cracking. Final Report

During the past year the authors have implemented and have begun validating a new procedure for obtaining partial charge distributions for use in molecular mechanics as well as for use in other methodologies that require a rapid charge estimation procedure. The new procedure is called QEq2. In addition, they have formulated a way of estimating resonance energies within molecular mechanics. This will permit screening transition state energetics as well as transition state structures for reactions involving eolites and hydrocarbon oxidation. Finally, during the past year, in collaboration with others they have begun an assessment of which of a variety of ab initio electronic structure methods can reproduce the known energetics of the reactions between hydrocarbons and metal oxo complexes. The major developments in the past year are summarized