S. Yashonath

Molecular Simulation of Condensed Matter
In early 1980s, significant advances were made in techniques for computer simulation. One such advance was the ability to simulate phase transformations in solids due to variable shape simulation cell at constant temperature and pressure. One of the problems encountered in studies of polyatomic systems in the proposed method was the rotation of the simulation cell. A simple yet effective solution to this was proposed[1,2] which is now routinely used.
A number of phase transformations in molecular solids have been studied using these techniques. CF4, CCl4, adamantane, biphenyl and toluene have been investigated[3]. High pressure studies on adamantane and the crystal to plastic crystal tranistion is being investigated.
In electrical engineering, there exist highly developed methods for analysis of electrical circuits. These methods can be employed in the analysis of chemical reactions if an equivalent electrical network can be written for the set of chemical reactions under consideration. Such an analysis was carried out for a few reactions.[4]
Role of reoritentation towards increase in the intermolecular interaction energy on vitrification in molecular glasses such as isomers of pentane have been investigated and its magnitude estimated. The results suggest that the contribution varies as a function of the shape. Globular isomer (neopentane) shows maximum contribution while linear(n-pentane) makes the least contribution[5-7].
There has been an enormous interest in the past decade towards confined fluids. In this laboratory, there has been a continued interest in the properties of confined fluids and understanding the various factors that determine them[8-10]. Most of the investigations focus on understanding diffusion within the porous solids. It has been found that, initially, the diffusion coefficient, D, decreases with sorbate size defined as σ the Lennard-Jones parameter. When the sorbate dimension approaches that of the void dimension, D exhibits a peak[11]. Such an anomalous behaviour arises from the peculiar situation where the force on the sorbate due to the host is nearly zero as a consequence of the mutual cancellation of the forces from the host wall[12]. For this reason, the effect has been called the Levitation effect. This is universal in that it is found in any porous solid irrespective of the pore geometry and topology, crystallianity[13] and even in solids dominated by long range Coulomb forces. It has recently been shown that this is the cause for the maximum in conductivity in nasicon structures. The results have ramifications in physics, chemistry (e.g., separation of hydrocarbons), materials science and biology (e.g., ion or molecule diffusion across biomembranes). Appropriate choice of the host so that the levitation parameter γ → 1 for one the components leads to excellent separation of mixtures as recent simulations have demonstrated[14].
The above results pertain to translational diffusion and the influence of the levitation effect on it. Recent work from this laboratory has shown that rotational motion may also be altered when the levitation parameter γ approaches unity, the condition under which the force on the guest is nearly zero. Translational-orientational coupling, orientational preference and partial freezing of rotational degrees of freedom. have been observed[15]. We have observed other interesting behaviours in these systems as well[16-20].
Dependence of self diffusivity on concentration has been investigated using the lattice gas model. The results of these studies not only reproduce the five known types of D vs. c dependence observed in PFG-NMR experiments but also report yet another new type. They further provide explanations and unify our present understanding of the concentration dependence of the D. PFG-NMR studies have found each system falls into one of these types. The present study, however, suggests that a given system may show more than one type of D vs. c behaviour depending on the external conditions. The type of behaviour observed can depend on the temperature.[21,22]
Non-equilibrium studies in the presence of inhomogeneous temperature distribution within porous solids and when γ << 1 and γ << 1 and γ 1 show interesting possibilities.
Molecular dynamics of binary mixtures in confinement to obtain the nature of self and distinct parts of diffusion has been carried out in detail. Making use of these, the distinct diffusivity Dd was computed for Ar/Kr mixture. Dd is significant when this mixture is confined to the pores of zeolite NaY as compared to bulk where Dd is zero.

Other areas of research are includes related aspects of what has been mentioned above.