Ashok K. Dham

Improved modelling of atom–molecule potential-energy surfaces: illustrative application to He–CO

The need for better potential-energy models for atom–molecule and molecule–molecule interactions is discussed and the utility of the exchange–coulomb (XC) model is critically examined, by fitting a potential based on it to new high-resolution discrete infrared data for the He–CO Van der Waals molecule. In addition to explaining the observed spectrum as well as does an optimized empirical potential previously determined from the same data, the resulting XC surface is expected to be more realistic in regions not directly sampled by the fitted data.

Multi-property predictions from recent He-CO potential energy surfaces and related comments on the nature of heteronuclear-rare gas interactions

Recently two potential energy surfaces, an empirical potential V (3,3,3) and a potential of the exchange-Coulomb (XC) form, have been determined by fitting their adjustable parameters to the observed line positions of the same high resolution infra-red spectra of He–CO. Both yield results for the IR spectra in excellent agreement with experiment and with each other but, interestingly, there are significant differences between the XC and V (3,3,3) potentials as a function of relative orientation and interspecies distance for all regions of configuration space, and not just for the repulsive walls. In this paper several properties of the He–CO dimer, second virial, binary diffusion and shear viscosity (both interaction and mixture) coefficients, are used to discriminate between XC, V (3,3,3) and several other literature potentials. The explicit calculations of the transport properties are carried out in the Mason–Monchik approximation and then scaled to give good estimates of close coupling classical trajec...

New exchange-Coulomb N2–Ar potential-energy surface and its comparison with other recent N2–Ar potential-energy surfaces

The reliability of five N2-Ar potential-energy surfaces in representing the N2-Ar interaction has been investigated by comparing their abilities to reproduce a variety of experimental results, including interaction second viral coefficients, bulk transport properties, relaxation phenomena, differential scattering cross sections, and the microwave and infrared spectra of the van der Waals complexes. Four of the surfaces are the result of high-level ab initio quantal calculations; one of them utilized fine tuning by fitting to microwave data. To date, these four potential-energy surfaces have only been tested against experimental microwave data. The fifth potential-energy surface, based upon the exchange-Coulomb potential-energy model for the interaction of closed-shell species, is developed herein: it is a combination of a damped dispersion energy series and ab initio calculations of the Heitler-London interaction energy, and has adjustable parameters determined by requiring essentially simultaneous agreement with selected quality interaction second viral coefficient and microwave data. Comparisons are also made with the predictions of three other very good literature potential-energy surfaces, including the precursor of the new exchange-Coulomb potential-energy surface developed here. Based upon an analysis of a large body of information, the new exchange-Coulomb and microwave-tuned ab initio potential-energy surfaces provide the best representations of the N2-Ar interaction; nevertheless, the other potential-energy surfaces examined still have considerable merit with respect to the prediction of specific properties of the N2-Ar van der Waals complex. Of the two recommended surfaces, the new exchange-Coulomb surface is preferred on balance due to its superior predictions of the effective cross sections related to various relaxation phenomena, and to its reliable, and relatively simple, representation of the long-range part of the potential-energy surface. Moreover, the flexibility still inherent in the exchange-Coulomb potential form can be further exploited, if required, in future studies of the N2-Ar interaction.

Empirical relation for the higher-order contributions to the viscosity of binary gas mixtures

An empirical relationship is given for the calculation of the higher-order contributions to the viscosity of binary gas mixtures in the Chapman-Cowling approach. A maximum error of five parts in ten thousands is observed.

An exchange-Coulomb model potential energy surface for the Ne-CO interaction. II. Molecular beam scattering and bulk gas phenomena in Ne-CO mixtures

Four potential energy surfaces are of current interest for the Ne-CO interaction. Two are high-level fully ab initio surfaces obtained a decade ago using symmetry-adapted perturbation theory and supermolecule coupled-cluster methods. The other two are very recent exchange-Coulomb (XC) model potential energy surfaces constructed by using ab initio Heitler-London interaction energies and literature long range dispersion and induction energies, followed by the determination of a small number of adjustable parameters to reproduce a selected subset of pure rotational transition frequencies for the (20)Ne-(12)C(16)O van der Waals cluster. Testing of the four potential energy surfaces against a wide range of available experimental microwave, millimeter-wave, and mid-infrared Ne-CO transition frequencies indicated that the XC potential energy surfaces gave results that were generally far superior to the earlier fully ab initio surfaces. In this paper, two XC model surfaces and the two fully ab initio surfaces are tested for their abilities to reproduce experiment for a wide range of nonspectroscopic Ne-CO gas mixture properties. The properties considered here are relative integral cross sections and the angle dependence of rotational state-to-state differential cross sections, rotational relaxation rate constants for CO(v=2) in Ne-CO mixtures at T=296 K, pressure broadening of two pure rotational lines and of the rovibrational lines in the CO fundamental and first overtone transitions at 300 K, and the temperature and, where appropriate, mole fraction dependencies of the interaction second virial coefficient, the binary diffusion coefficient, the interaction viscosity, the mixture shear viscosity and thermal conductivity coefficients, and the thermal diffusion factor. The XC model potential energy surfaces give results that lie within or very nearly within the experimental uncertainties for all properties considered, while the coupled-cluster ab initio surface gives results that agree similarly well for all but one of the properties considered. When the present comparisons are combined with the ability to give accurate spectroscopic transition frequencies for the Ne-CO van der Waals complex, only the XC potential energy surfaces give results that agree well with all extant experimental data for the Ne-CO interaction.

Steady state creep in a rotating composite disc of variable thickness

Abstract The steady state creep in a rotating disc having variable thickness and made of isotropic aluminium–silicon carbide particulate composite has been investigated. Using threshold stress based creep law, the general expressions for stresses and strain rates in the discs have been obtained. The expressions are used to calculate the distributions of stresses and strain rates in different discs viz (i) disc having constant thickness (ii) disc having linearly varying thickness and (iii) disc having hyperbolically varying thickness. The volume of different discs is kept the same. The study revealed that the stresses and strain rates in the disc could be reduced to a significant extent by varying the disc profile. The linearly varying disc exhibits the lowest values of stresses and strain rates compared to those observed in hyperbolic or uniform thickness disc.

An exchange-Coulomb model potential energy surface for the Ne–CO interaction. I. Calculation of Ne–CO van der Waals spectra

Exchange-Coulomb model potential energy surfaces have been developed for the Ne-CO interaction. The initial model is a three-dimensional potential energy surface based upon computed Heitler-London interaction energies and literature results for the long-range induction and dispersion energies, all as functions of interspecies distance, the orientation of CO relative to the interspecies axis, and the bond length of the CO molecule. Both a rigid-rotor model potential energy surface, obtained by setting the CO bond length equal to its experimental spectroscopic equilibrium value, and a vibrationally averaged model potential energy surface, obtained by averaging the stretching dependence over the ground vibrational motion of the CO molecule, have been constructed from the full data set. Adjustable parameters in each model potential energy surface have been determined through fitting a selected subset of pure rotational transition frequencies calculated for the (20)Ne-(12)C(12)O isotopolog to precisely known experimental values. Both potential energy surfaces provide calculated results for a wide range of available experimental microwave, millimeter-wave, and midinfrared Ne-CO transition frequencies that are generally far superior to those obtained using the best current literature potential energy surfaces. The vibrationally averaged CO ground state potential energy surface, employed together with a potential energy surface obtained from it by replacing the ground vibrational state average of the CO stretching dependence of the potential energy surface by an average over the first excited CO vibrational state, has been found to be particularly useful for computing and/or interpreting mid-IR transition frequencies in the Ne-CO dimer.

Investigating the effect of thickness profile of a rotating functionally graded disc on its creep behavior

Creep behavior of rotating discs made of functionally graded materials with linearly varying thickness has been investigated. The discs contain silicon carbide particles in a matrix of pure aluminum. The effect of varying disc thickness gradient (TG) has been investigated on the stresses and strain rates in the composite disc. The study shows that with the increase in disc TG, the radial, tangential and effective stresses decrease throughout the disc. The strain rates in the disc also reduce significantly with the increase in TG of the disc.

Transport coefficients of ternary gas mixtures

General expressions for the transport coefficients of ternary gas mixtures have been written in terms of the infinite matrices within the framework of the Chapman-Enskog method. Using these expressions, higher-order contributions to the various transport coefficients have been studied for the rigid sphere and the LJ (12-7) potential. These calculations indicate that the higher-order contributions decrease rapidly for all the coefficients.

Higher order contributions to the transport coefficients of binary gas mixtures

The rate of decrease of higher order corrections to various transport coefficients of binary gas mixtures has been established for a temperature range (200K<or=T<or=2000K) in the case of the LJ (12-7) potential. The authors study shows that, depending upon the experimental accuracy, at least second- order calculations for viscosity and third-order calculations for other coefficients of binary mixtures should be performed to check the validity of an intermolecular potential. In obtaining these results, general expressions for transport coefficients have been written in terms of the inverse of matrices. The utility of these expressions for computer programming has been indicated.