A molecular-dynamics algorithm for mixed hard-core/continuous potentials

We present a new molecular-dynamics algorithm for integrating the equations of motion for a system of particles interacting with mixed continuous/impulsive forces. This method, which we call Impulsive Verlet, is constructed using operator splitting techniques similar to those that have been used successfully to generate a variety molecular-dynamics integrators. In numerical experiments, the Impulsive Verlet method is shown to be superior to previous methods with respect to stability and energy conservation in long simulations.Comment: 18 pages, 6 postscript figures, uses rotate.st

Constant-temperature molecular-dynamics algorithms for mixed hard-core/continuous potentials

We present a set of second-order, time-reversible algorithms for the isothermal (NVT) molecular-dynamics (MD) simulation of systems with mixed hard-core/continuous potentials. The methods are generated by combining real-time Nose' thermostats with our previously developed Collision Verlet algorithm [Mol. Phys. 98, 309 (1999)] for constant energy MD simulation of such systems. In all we present 5 methods, one based on the Nose'-Hoover [Phys. Rev. A 31, 1695 (1985)] equations of motion and four based on the Nose'-Poincare' [J.Comp.Phys., 151 114 (1999)] real-time formulation of Nose' dynamics. The methods are tested using a system of hard spheres with attractive tails and all correctly reproduce a canonical distribution of instantaneous temperature. The Nose'-Hoover based method and two of the Nose'-Poincare' methods are shown to have good energy conservation in long simulations.Comment: 9 pages, 5 figure

A Combined Experimental-Computational Study on the Effect of Topology on Carbon Dioxide Adsorption in Zeolitic Imidazolate Frameworks

We report CO₂ adsorption data for four zeolitic imidazolate frameworks (ZIFs) to 55 bar, namely ZIF-7, ZIF-11, ZIF-93, and ZIF-94. Modification of synthetic conditions allows access to different topologies with the same metal ion and organic link: ZIF-7 (ZIF-94) having <b>sod</b> topology and ZIF-11 (ZIF-93) having the <b>rho</b> topology. The varying topology, with fixed metal ion and imidazolate functionality, makes these systems ideal for studying the effect of topology on gas adsorption in ZIFs. The experiments show that the topologies with the smaller pores (ZIF-7 and 94) have larger adsorptions than their counterparts (ZIF-11 and 93, respectively) at low pressures (<1 bar); however, the reverse is true at higher pressures where the larger-pore structures have significantly higher adsorption. Molecular modeling and heat of adsorption measurements indicate that while the binding potential wells for the smaller-pore structures are deeper than those of the larger-pore structures, they are relatively narrow and cannot accommodate multiple CO₂ occupancy, in contrast to the much broader potential wells seen in the larger pore structures