Quasi-chemical theory with a soft cutoff

In view of the wide success of molecular quasi-chemical theory of liquids, this paper develops the soft-cutoff version of that theory. This development has important practical consequences in the common cases that the packing contribution dominates the solvation free energy of realistically-modeled molecules because treatment of hard-core interactions usually requires special purpose simulation methods. In contrast, treatment of smooth repulsive interactions is typically straightforward on the basis of widely available software. This development also shows how fluids composed of molecules with smooth repulsive interactions can be treated analogously to the molecular-field theory of the hard-sphere fluid. In the treatment of liquid water, quasi-chemical theory with soft-cutoff conditioning doesn't change the fundamental convergence characteristics of the theory using hard-cutoff conditioning. In fact, hard cutoffs are found here to work better than softer ones.Comment: 5 pages, 2 figure

Quantum Mechanical Single Molecule Partition Function from Path Integral Monte Carlo Simulations

An algorithm for calculating the partition function of a molecule with the path integral Monte Carlo method is presented. Staged thermodynamic perturbation with respect to a reference harmonic potential is utilized to evaluate the ratio of partition functions. Parallel tempering and a new Monte Carlo estimator for the ratio of partition functions are implemented here to achieve well converged simulations that give an accuracy of 0.04 kcal/mol in the reported free energies. The method is applied to various test systems, including a catalytic system composed of 18 atoms. Absolute free energies calculated by this method lead to corrections as large as 2.6 kcal/mol at 300 K for some of the examples presented

Nonequilibrium molecular dynamics simulations of diffusion of binary mixtures containing short η-alkanes in faujasite

We have used nonequilibrium molecular dynamics (NEMD) simulations to evaluate diffusivities for binary mixtures of methane/CF<SUB>4</SUB>, propane/CF<SUB>4</SUB>, η-butane/CF<SUB>4</SUB>, and n-butane/ethane in zeolite faujasite at 300 K. A formula to estimate the error bars in the transport coefficients from NEMD is also presented. NEMD simulations can give cross coefficients of the Onsager matrix with considerably smaller error bars than those obtained from equilibrium MD (EMD). We evaluated diffusion coefficients that could not be evaluated previously using EMD. An estimation scheme based on the Maxwell-Stefan formulation was tested for predicting multicomponent diffusivities based on single component diffusion data. This estimation scheme works very well for the systems tested