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We report molecular-dynamics simulations of Pd:H to elucidate transport properties, with special focus placed on determining the temperature dependence of the heat of transport Q Ã. Simulation results are analyzed using the Green-Kubo approach. It is found that Q Ã describing the thermodiffusion of hydrogen increases linearly with temperature. By contrast, the reduced heat of transport Q Ã0 ¼ Q Ã À h 2 , with h 2 the partial enthalpy of hydrogen, is approximately temperature independent. By computing separately the potential, kinetic, and virial contributions to Q Ã , it is possible to understand key features of the thermodiffusion process. In particular, the sum of the kinetic and potential energy of hydrogen atoms is increased above that of an average hydrogen atom by an amount comparable to the migration energy during a successful hop. However, the virial term in the energy flux is less than what would be expected based on the average local stress contribution due to the hydrogen atoms. Detailed calculations show that the relevant component of the stress tensor due to a hopping hydrogen atom exhibits a minimum at the transition state. Hence, while Q Ã has significant positive contributions due to the excited nature of the hopping hydrogen atom, the reduced heat of transport Q Ã0 can still be negative. The results here present important insight into the failure of simple kinetic theories of thermodiffusion, and provide a new perspective that can be tested on other systems. V

The accretion of dust grains to form larger objects, including planetesimals, is a central problem in planetary science. It is generally thought that weak van der Waals interactions play a role in accretion at small scales where gravitational attraction is negligible. However, it is likely that in many instances, chemical reactions also play an important role, and the particular chemical environment on the surface could determine the outcomes of dust grain collisions. Using atomic-scale simulations of collisional aggregation of nanometer-sized silica (SiO2) grains, we demonstrate that surface hydroxylation can act to weaken adhesive forces and reduce the ability of mineral grains to dissipate kinetic energy during collisions. The results suggest that surface passivation of dangling bonds, which generally is quite complete in an Earth environment, should tend to render mineral grains less likely to adhere during collisions. It is shown that during collisions, interactions scale with interparticle distance in a manner consistent with the formation of strong chemical bonds. Finally, it is demonstrated that in the case of collisions of nanometer-scale grains with no angular momentum, adhesion can occur even for relative velocities of several kilometers per second. These results have significant implications for early planet formation processes, potentially expanding the range of collision velocities over which larger dust grains can form.