Evidence of scaling in the high pressure phonon dispersion relations of some elemental solids.

First principles searches are carried out for the existence of an asymptotic scaling law for the zero temperature phonon dispersion relation of several elemental crystalline solids in the high pressure regime. The solids studied are Cu, Ni, Pd, Au, Al, and Ir in the face-centered-cubic (fcc) geometry and Fe, Re, and Os in the hexagonal-close-packed (hcp) geometry. At higher pressures, the dependence of the scale of frequency on pressure can be fitted well by a power law. Elements with a given crystalline geometry have values of the scaling exponent very close to each other (0.32 for fcc and 0.27 for hcp - with a scatter below five percent of the average).

Wang-Landau density of states based study of the folding-unfolding transition in the mini-protein Trp-cage (TC5b).

We present the results of a high-statistics equilibrium study of the folding/unfolding transition for the 20-residue mini-protein Trp-cage (TC5b) in water. The ECEPP/3 force field is used and the interaction with water is treated by a solvent-accessible surface area method. A Wang-Landau type simulation is used to calculate the density of states and the conditional probabilities for the various values of the radius of gyration and the number of native contacts at fixed values of energy--along with a systematic check on their convergence. All thermodynamic quantities of interest are calculated from this information. The folding-unfolding transition corresponds to a peak in the temperature dependence of the computed specific heat. This is corroborated further by the structural signatures of folding in the distributions for radius of gyration and the number of native contacts as a function of temperature. The potentials of mean force are also calculated for these variables, both separately and jointly. A local free energy minimum, in addition to the global minimum, is found in a temperature range substantially below the folding temperature. The free energy at this second minimum is approximately 5 k(B)T higher than the value at the global minimum.

Universality in the vibrational spectra of single-component amorphous clusters.

We have numerically investigated the vibrational spectra of amorphous single-component clusters for several types of interactions among the particles. For all the potentials we have studied, we find that the density of states can be described, except at the two ends of the spectrum, by the same functional form to a very good approximation, and that the fluctuation properties of the spectra in this central region converge to those of the Gaussian orthogonal ensemble of random matrices with increasing system size. We conjecture that this scenario is true for a broad class of potentials.