Structure of phosphorus-selenium glasses: results from ab initio molecular dynamics simulations

Car-Parrinello molecular dynamics calculations have been carried out for phosphorus-selenium mixtures of compositions corresponding to overall stoichiometrics PSe and P₂Se. Comparison of the calculated structure factors with those obtained in neutron scattering experiements suggests that the real materials are characterized by the presence of substantial numbers of well defined molecular units.     

Constant pressure-constant temperature molecular dynamics: a correct constrained NPT ensemble using the molecular virial

In the present work we introduce a simple, Nosé—Hoover style isothermal—isobaric molecular dynamics method for systems with holonomic molecular constraints and the molecular representation of the virial. We prove, using the non-Hamiltonian dynamics approach, recently developed by Tuckerman et al. [1999, Europhys. Lett., 45, 149], that the phase space distribution, generated by our equations, samples the desired ensemble under all circumstances. We also write down the explicit reversible integrator for our equations. This integrator has been implemented in the last version of the DLProtein program.     

Car—Parrinello study of Ziegler—Natta heterogeneous catalysis: stability and destabilization problems of the active site models

Recent results on Car—Parrinello simulations of MgCl₂/TiCl₄ Ziegler—Natta heterogeneous catalytic systems are reviewed and the stability of Ti active site configurations on the various active surfaces are discussed. In particular, the focus is on the ability of an active centre to carry out the polymerization process efficiently, achieved by monitoring geometrical changes and associated energetics during the reaction. The active site geometry is crucial in enhancing or decreasing the activation barrier, in selecting the olefin enantioface and in some cases in destabilizing the catalytic centre. A key point in the binding/destabilization interplay is the matching of Ti with the substrate, an issue that represents a fundamental difference with respect to homogeneous systems.