Efficient Monte Carlo Simulations of Gas Molecules Inside Porous Materials

Monte Carlo (MC) simulations are commonly used to obtain adsorption properties of gas molecules inside porous materials. In this work, we discuss various optimization strategies that lead to faster MC simulations with CO 2 gas molecules inside host zeolite structures used as a test system. The reciprocal space contribution of the gas-gas Ewald summation and both the direct and the reciprocal gas-host potential energy interactions are stored inside energy grids to reduce the wall time in the MC simulations. Additional speedup can be obtained by selectively calling the routine that computes the gas-gas Ewald summation, which does not impact the accuracy of the zeolites adsorption characteristics. We utilize two-level density-biased sampling technique in the grand canonical Monte Carlo (GCMC) algorithm to restrict CO 2 insertion moves into low-energy regions within the zeolite materials to accelerate convergence. Finally, we make use of the graphics processing units (GPUs) hardware to conduct multiple MC simulations in parallel via judiciously mapping the GPU threads to available workload. As a result, we can obtain a CO 2 adsorption isotherm curve with 14 pressure values (up to 10 atm) for a zeolite structure within a minute of total compute wall time. © 2012 American Chemical Society.

Molecular Monte Carlo Simulations Using Graphics Processing Units: To Waste Recycle or Not?

Ji Hyun Kim, Berend Smit

In the waste recycling Monte Carlo (WRMC) algorithm,(1)multiple trial states may be simultaneously generated and utilized during Monte Carlo moves to improve the statistical accuracy of the simulations, suggesting that such an algorithm may be well posed for implementation in parallel on graphics processing units (GPUs). In this paper, we implement two waste recycling Monte Carlo algorithms in CUDA (Compute Unified Device Architecture) using uniformly distributed random trial states and trial states based on displacement random-walk steps, and we test the methods on a methane-zeolite MFI framework system to evaluate their utility. We discuss the specific implementation details of the waste recycling GPU algorithm and compare the methods to other parallel algorithms optimized for the framework system. We analyze the relationship between the statistical accuracy of our simulations and the CUDA block size to determine the efficient allocation of the GPU hardware resources. We make comparisons between the GPU and the serial CPU Monte Carlo implementations to assess speedup over conventional microprocessors. Finally, we apply our optimized GPU algorithms to the important problem of determining free energy landscapes, in this case for molecular motion through the zeolite LTA. © 2011 American Chemical Society.

2011

Adsorption Thermodynamics and Intrinsic Activation Parameters for Monomolecular Cracking of n-Alkanes on Bronsted Acid Sites in Zeolites

Berend Smit

Experimental' measurements of the rate coefficient (k(app)) and apparent enthalpies and entropies of activation (Delta H-app arid Delta S-app) for alkane cracking catalyzed by acidic zeolites can be used to characterize the effects of zeolite structure and alkane size On the intrinsic enthalpy and entropy of activation, Delta H-int(double dagger) and Delta S-int(double dagger). To determine Delta H-int(double dagger) and Delta S-int(double dagger), enthalpies and entropies of adsorption, Delta Hads-H+ and Delta Sads-H+, must be determined for alkane molecules moving from the gas phase to Bronsted add sites at reaction temperatures (>673 K). Experimental values of Delta H-app and Delta S-app must also be properly defined in terms of Delta Hads-H+ and Delta Hads-H+. We report here a method for determining Delta Hads-H+ and Delta Sads-H+ in which the adsorption site is represented by a fixed volume that includes the proton. Values of Delta Hads-H+ and Delta Sads-H+ obtained from Monte Carlo simulations are in good agreement with values obtained from experimental data measured at 300-400 K. An important feature of the simulations, however, is their ability to account for the redistribution of alkane adsorbed at protons in different locations with ineteasing temperature. Values of Delta H-int(double dagger) and Delta S-int(double dagger), for the cracking of propane through n-hexane, determined from measured values of k(app) and Delta H-app and simulated: values of Delta Hads-H+ and Delta Sads-H+, agree well with values obtained independently from quantum mechanics/molecular mechanics calculations. Application of our method of analysis reveals that the observed increase in kapp with increasing n-alkane size is due primarily to a decrease in Delta H-int(double dagger) with increasing chain length and that Delta S-int(double dagger) is independent of chain length.

American Chemical Society2015

Adsorption Thermodynamics and Intrinsic Activation Parameters for Monomolecular Cracking of n-Alkanes on Bronsted Acid Sites in Zeolites

Berend Smit

American Chemical Society2015