Kamakshi Jagannathan

Monte Carlo simulations for the phase behavior of symmetric nonadditive hard sphere mixtures

Computer simulation results are presented for the phase behavior of a symmetric binary mixture of nonadditive hard spheres. In this model, the hard sphere diameters are given by σAA=σBB=λd and σAB=d. At high densities and for small enough λ, this hard sphere mixture exhibits a fluid–fluid phase separation into an A-rich and a B-rich phase. Semigrand ensemble simulations are performed for the critical point and the phase behavior of this model for various values of λ. The results for the critical density are significantly different from previous simulation estimates. A comparison of our simulation results to existing theories shows that none of the theories are accurate for the location of the critical point, over the entire range of λ.

Density functional theory and Monte Carlo simulations for hard sphere fluids in square and rectangular channels

The density distribution of hard spheres in square and rectangular channels is studied using density functional theory and grand canonical ensemble Monte Carlo simulations. The theory uses the weighted density approximation for the excess Helmholtz free energy functional with the Curtin–Ashcroft approximation for the weighting function. The theoretical predictions are in good quantitative agreement with the simulations except for very high densities. The theory predicts pronounced layering in the channel as the density is increased, with high values of density at the surfaces and even higher densities at the corners where any two surfaces meet. Interesting interference effects are observed in the density profiles when compared to the case of a hard sphere fluid in slitlike pores.

A Monte Carlo study of the self-assembly of Bacteriorhodopsin

Bacteriorhodopsin (BR) and specific lipid molecules self-assemble into a quasi two-dimensional lattice structure known as the purple membrane (PM). In the PM, BR molecules exist in a trimeric form with lipid molecules present in the space enclosed by each trimeric unit and in the inter-trimer space. These trimeric units, which have a roughly circular cross-section, are arranged in hexagonal patterns with long-ranged crystalline order. In this work, we investigate the self-assembly of BR in the PM via Monte Carlo simulations of a two-dimensional model of the membrane and proteins. The protein molecules are modeled as 120 degrees sectors of a circle and the lipid molecules enter into the model through effective protein-protein interactions. The sectors cannot overlap with each other, and in addition to this excluded volume interaction there are site-site attractive interactions between specific points of the proteins to mimic interactions between helices on the proteins and lipid-induced interactions. At low values of the attractive well depth, the proteins are found in the monomeric form at all concentrations. At moderate and high values of the attractive well depth, trimers are formed as the concentration increases, and with a further increase in concentration the trimers organize into a hexagonal lattice. The interactions between the proteins and those induced by the intra-trimer lipids play an equally important role in the formation of trimers and the lattice. The lipids in the inter-trimer space cause the trimers to orient in a specific direction in the hexagonal crystal lattice.

Dynamics of fluids near the consolute critical point: a molecular-dynamics study of the Widom-Rowlinson mixture.

Molecular-dynamics simulations are presented for the dynamic behavior of the Widom-Rowlinson mixture [B. Widom, and J. S. Rowlinson, J. Chem. Phys. 52, 1670 (1970)] at its critical point. This model consists of two components where like species do not interact and unlike species interact via a hard-core potential. Critical exponents are obtained from a finite-size scaling analysis. The self-diffusion coefficient shows no anomalous behavior near the critical point. The shear viscosity and thermal conductivity show no divergent behavior for the system sizes considered, although there is a significant critical enhancement. The mutual diffusion coefficient, D(AB), vanishes as D(AB) approximately xi(-1.26 +/- 0.08), where xi is the correlation length. This is different from the renormalization-group (D(AB) approximately xi(-1.065)) mode coupling theory (D(AB) approximately xi(-1)) predictions. The theories and simulations can be reconciled if we assume that logarithmic corrections to scaling are important.