Emeritus Professor
Ph.D. IISc Bangalore
Phone: +91 80 2293 2568

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.

  1. S. Yashonath and C.N.R. Rao, Monte Carlo simulation of crystal to plastic crystal transition in carbon tetrachloride, Chem. Phys. Lett. 119(1985) 22.
  2. S. Yashonath and C.N.R. Rao, A Monte Carlo study of crystal structure transformations, Mol Phys. 54 (1985) 245.
  3. C.N.R. Rao and S. Yashonath, Computer simulation of phase transformations in solids, J. Solid State Chem. 68 (1987) 193.
  4. S. Yashonath, Relaxation time of chemical reactions from network thermodynamics, J. Phys. Chem. 45(1981) 1808.
  5. Aparna Chakrabarti, S. Yashonath and C.N.R. Rao, Importance of orientational rearrangement during vitrification of hydrocarbons: dependence on molecular shape, J. Phys. Chem. 96 (1992) 6762.
  6. Aparna Chakrabarti, S. Yashonath and C.N.R. Rao, Comparision of positional disorder in the liquid and glassy states of hydrocarbons: dependence of disorder on molecular shape, Mol. Phys. 81 (1994) 467.
  7. S. Yashonath and V.C. Jyothi Bhasu, Structure-potential relationship and nature of disorder in liquids and glassy solids, Chem. Phys. Lett. 189 (1992) 311.
  8. S. Yashonath, Adsorption of xenon in zeolite Y: a molecular dyanmics study, Chem. Phys. Lett. 177 (1991) 54.
  9. S. Yashonath, J. M. Thomas,  A. K. Nowak,  A. K. Cheetham,  The siting, energetics and mobility of saturated-hydrocarbons inside zeolitic  cages – methane in zeolite-Y, Nature331(6157), (1988), 601.
  10. S. Yashonath and P. Santikary, Influence of non-geometrical factors on the intracrystalline diffusion: importance of the sorbate-zeolite interactions, Mol. Phys. 78 (1993) 1.
  11. S. Yashonath and P. Santikary, Diffusion of sorbates in zeolites Y and A: Novel dependence on sorbate size and on strength of sorbate-zeolite interaction, J. Phys. Chem. 98 (1994) 6368.
  12. M. Ghosh, G. Ananthakrishna, S. Yashonath, P. Demontis and G. Suffrtti, Probing potential energy surfaces in confined systems: Behaviour of mean square displacement in zeolites, J. Phys. Chem. 98 (1994) 9354.
  13. R. Chitra and S. Yashonath, Separation of multi-component mixtures by the use of the anomalous regime in the diffusivity, Mol. Phys. 98 (2000) 657.
  14. R. Chitra, A. V. Anil Kumar and S. Yashonath, Translational-orientational coupling during the passage of methane through the bottleneck in zeolite A, J. Chem. Phys.(Commun.) 114 (2001) 11.
  15. S. Yashonath and P. Santikary, Xenon in sodium Y zeolite. 2. Arrhenius relation, mechanism and barrier height distribution for cage-to-cage diffusion, J. Phys. Chem. 97 (1993) 3849.
  16. M. Ghosh, G. Ananthakrishna, S. Yashonath, P. Demontis and G. Suffrtti, Probing potential energy surfaces in confined systems : Behaviour of mean square displacement in zeolites, J. Phys. Chem. 98(1994) 9354.
  17. B. Bhattacharjee and S. Yashonath, Dependence of diffusion properties in zeolites Y and A: A search in the sorbate interaction parameter space, Mol. Phys. 90(1997) 889.
  18. Sanjoy Band.yopadhyay and S. Yashonath, Conformational properties of n-butane in zeolite Y, J.Chem. Phys. 105(1996) 7223.
  19. R. Chitra and S. Yashonath, Estimation of error in the diffusion coefficient from molecular dynamics simulations, J. Phys. Chem. B101 (1997) 5437.
  20. S. Y. Bhide and S. Yashonath, Dependence of the self-diffusion coefficient on the sorbate concentration : A two dimensional lattice gas model with and without confinement, J.Chem. Phys. 111. (1999) 6658.
  21. S. Y. Bhide and S. Yashonath, Types of dependence of self-diffusivity on sorbate concentration in parameter space: A two dimensional lattice gas study, J. Phys. Chem. B104 (2000) 2607.
  22. V. Anil Kumar and S. Yashonath, Effect of a distribution of pore dimension on levitation effect, J. Phys. Chem. B104 (2000) 9126.
  23. A.V. Anil Kumar, S. Yashonath and S. L. Chaplot, A study of the condensed phases and solid-solid phase transition in tolune: A Monte Carlo investigation, J.Chem. Phys. 113, (2000), 8070.
  24. S.Y. Bhide and S. Yashonath, Structure and dynamics of benzene in one-dimensional channels, J. Phys. Chem. 104(2000) 11977.
  25. R. Chitra, A. V. Anil Kumar and S. Yashonath, Translational-orientational coupling during the passage of methane through the bottleneck in zeolite A, J. Chem. Phys.(Commun.) 114 (2001) 11.
  26. P. Padma Kumar and S. Yashonath, Lithium ion motion in LiZr2(PO4)3, J. Phys. Chem. B105 (2001) 6785.
  27. V. Anil Kumar, S. Yashonath, M. Sluiter and Y. Kawazoe, Rotational motion of methane within the confines of zeolite NaCaA: Molecular dynamics and ab initio calculations, Phys. Rev. E 65, 011203 (2002).
  28. P.~P. Kumar; S. Yashonath,  “A full interionic potential for Na1+xZr2SixP3-xO12 superionic conductors”, J. Am. Chem. Soc., 124,  (2002), 3828,.