No abstract

No abstract

No abstract

Equilibrium and non-equilibrium molecular dynamics techniques are used to simulate a molecular liquid (n-hexadecane) at extreme physical conditions (high temperatures, pressures and shear rates), typical of those encountered in many elastohydrodynamic lubrication applications, and to study the effects of pressure and temperature on the calculated rheological properties. Pressure and temperature effects are found to be more pronounced at low shear rates than at high shear rates, where the non-Newtonian behaviour is dominated by the strong external kinematic effects. The observed non-Newtonian behaviour of the pressure and temperature coeficients of viscosity and the variation of certain molecular geometric functions calculated to assist the microscopic understanding are combined to provide a possible explanation for the phenomenon of limiting shear stress under elastohydrodynamic lubrication conditions. Further implications for the behaoiour of lubricant-like molecules in practical applications are also discussed.

Dedicated to Dr. K. Heinzinger on the occasion of his 60 th birthdayThe derivation of an intermolecular potential function for the interaction of two hydroxylamine molecules, based on ab initio SCF-ECP calculations is reported. Ab initio values were compared for selected geometries with those obtained from calculations with much larger basis sets, inluding electron correlation effects by the CEPA method. A total 658 energy values were then fitted to an analytic sum of atom-atom isotropic pair potential functions, whose functional form was given a simple electrostatic interpretation. The major difficulties, arising from the relatively low values of the stabilisation energies of the system and the numerous possibilities to form hydrogen bonds, were overcome by a careful selection of sufficient points on the potential hypersurface, introduction of cut-offs and weight factors. The obtained function was seen to be able to give a good reproduction of the interaction energies and proved suitable in a preliminary MC simulation.I n te r m o le c u la r P o te n tia l F u n c tio n fo r H y d r o x y la m in e D im e r I n te r a c tio n s fro m a b in itio C a lc u la tio n s IntroductionFor the computer simulation of liquids [1], realistic pair potentials between the molecules involved are needed.In this note we report the construction of a poten tial function for the interaction between two hydroxyl amine molecules based on ab initio SCF calculations. Hydroxylamine has been chosen for several reasons. First, being still a relatively small molecule, it allows the application of sufficient accurate ab initio calcula tions. Second, chemically situated between water and ammonia [2] and being a good solvent for electrolytes, MC and/or MD simulations of ions in hydroxylamine as solvent will be of particular interest for compari sons with similar simulations for water [3], ammonia [4] and water/ammonia mixtures [5] as solvents. Third, the possibility of forming several types of hydrogen bonds and the existence of two possible coordination sites for ions make this molecule an especially versatile and challenging example for a theoretical approach, as experimental methods for structural investigations of hydroxylamine solutions meet several restrictions that could be overcome by the quantum chemical and statistical simulation treatment.Reprint requests to Prof. Dr. B. M. Rode, Institut für Anorg. und Analytische Chemie, Universität Innsbruck, Innrain 52a. A-6020 Innsbruck. Österreich. Method Monomer Geometry and Basis SetAs a first step, full geometry optimisations by the force field method have been carried out for hydroxyl amine with various basis sets. As the results collected in Table 1 show, considerably diverging data are ob tained, and even the best of these standard basis sets do not lead to full agreement with experimental data [6]. Moreover, it could be expected from results for hydrogen peroxide and hydrazine [7], that electron correlation will be a non-negligible factor for the determination of the equilibrium geome...