Shear-induced changes of the potential energy landscape underlying n-butane liquids and glasses

The effects of shear strain on n-butane liquids and glasses are examined wi th molecular simulations. The simulations focus on the nonlinear mechanical properties and the transport properties of these systems. These properties are analyzed in terms of the changes in the potential energy landscape wit h strain, which is quantified by the curvature of the energy minima and the height, position, and curvature of barriers between minima. Shear strain i s shown to cause local minima to disappear and barrier heights to decrease to zero, which renders the system mechanically unstable and forces the syst em toward alternate local minima. These disappearances of energy minima and barriers lead to plastic deformation in mechanically stressed n-butane gla sses, and enhanced diffusion and molecular alignment in flowing n-butane li quids. (C) 2000 American Institute of Physics. [S0021-9606(00)50506-6].