Chemical Physics

We study the thermodynamic behavior of the random chain model proposed by Iori, Marinari and Parisi, and how this depends on the actual sequence of interactions along the chain. The properties of randomly chosen sequences are compared to those of designed ones, obtained through a simulated annealing procedure in sequence space. We show that the transition to the folded phase takes place at a smaller strength of the quenched disorder for designed sequences. As a result, folding can be relatively fast for these sequences.

Non-adiabatic molecular dynamics simulations are used to analyze the role of different solvent degrees of freedom in the non-radiative relaxation of the first excited state of the hydrated electron. The relaxation occurs through a multi-mode coupling between the adiabatic electronic states. The process cannot be described by a single mode promotion model frequently used in the ``large molecule'' limit of gas phase theories. Solvent librations and vibrations, and the H2O asymmetric stretch in particular, are found to be the most effective promotors of the electronic transition. Dissipation of the released energy to the solvent proceeds on two time scales: a fast 10-20 fs heating of the first solvation shell, where most of the energy is accepted by the librational degrees of freedom, and a several hundred femtosecond global reconstruction of t he solvent as the first shell transfers its excess energy to the rest of the molecules. The implications of our use of a semiclassical approximation as the criterion for good promoting and accepting modes are discussed.

The thermodynamic properties such as the specific heat are uniquely determined by the second moments of the energy distribution for a given ensemble averaging. However for small particle numbers the results depend on the ensemble chosen. We calculated the higher moments of the distributions of some observables for both the canonical and the microcanonical ensemble of the same van der Waals clusters. The differences of the resulting thermodynamic observables for the two ensembles are calculated in terms of the higher moments. We demonstrate how for increasing particle number these terms decrease to vanish for bulk material. For the calculation of the specific heat within the microcanonical ensemble we give a new method based on an analysis of histograms.

The shifts of the electronic absorption spectra of GaAs and GaP semiconductor clusters are calculated using accurate pseudopotentials. In the absence of experimental data at present, these calculations provide estimates of expected spectral shifts in these clusters. In addition, these calculations show that Coulomb interaction between the electron and hole dominates over the confinement energy in small clusters, with the result that the electronic absorption spectra of small clusters exhibit redshift instead of blueshift as the cluster size is decreased.

A new spectrum generating algebra for a unified description of rotations and vibrations in polyatomic molecules is introduced. An application to nonlinear X 3 molecules shows that this model (i) incorporates exactly the relevant point group, (ii) provides a complete classification of oblate top states, and (iii) treats properly both degenerate and nondegenerate vibrations.

We study {{\rm C} 60 } with the use of Thomas-Fermi theory. A spherical shell model is invoked to treat the nuclear potential, where the nuclear and core charges are smeared out into a shell of constant surface charge density. The valence electron distribution and the electrostatic potential are efficiently computed by integration of the Thomas-Fermi equation, subject to the shell boundary conditions. Total energy is numerically calculated over a range of shell radii, and the mechanical stability of the model is explored, with attention to the constraints of Teller's theorem. The calculated equilibrium radius of the shell is in good agreement with experiment.

We present a simple proof of the stability of the hydrogen molecule ( M + M + m − m − ) . It does not rely on the proton-to-electron mass ratio M/m being very large, and actually holds for arbitrary values of M/m . Some asymmetric molecules of the type ( m + 1 m + 2 m − 3 m − 4 ) are also stable. Possible applications to molecules containing antiparticles and to exotic hadrons in the quark model are briefly outlined. Revtex Version 3.0, 4 Figures available on request by Fax or Mail.

Tight-binding molecular dynamic simulations have revealed that Si 12 is an icosahedron with all atoms on the surface of an approximately 5 Å~ diameter sphere. This is the most spherical cage structure for silicon clusters in the 2-13 atom size range.

We determined the structures of silicon clusters in the 11-14 atom size range using the tight-binding molecular dynamics method. These calculations reveal that \Si{11} is an icosahedron with one missing cap, \Si{12} is a complete icosahedron, \Si{13} is a surface capped icosahedron, and \Si{14} is a 4-4-4 layer structure with two caps. The characteristic feature of these clusters is that they are all surface.

We study the thermodynamic behavior of a simple off-lattice model for protein folding. The model is two-dimensional and has two different ``amino acids''. Using numerical simulations of all chains containing eight or ten monomers, we examine the sequence dependence at a fixed temperature. It is shown that only a few of the chains exist in unique folded state at this temperature, and the energy level spectra of chains with different types of behavior are compared. Furthermore, we use this model as a testbed for two improved Monte Carlo algorithms. Both algorithms are based on letting some parameter of the model become a dynamical variable; one of the algorithms uses a fluctuating temperature and the other a fluctuating monomer sequence. We find that by these algorithms one gains large factors in efficiency in comparison with conventional methods.