Distribution functions of a simple fluid under shear: Low shear rates

Anisotropic pair distribution functions for a simple, soft sphere fluid at moderate and high density under shear have been calculated by nonequilibriu m molecular dynamics, by equilibrium molecular dynamics with a nonequilibri um potential, and by a nonequilibrium distribution function theory [H. H. C an and B. C. Eu, Phys. Rev. A 45, 3670 (1992)] and some variants. The noneq uilibrium distribution function theory consists of a nonequilibrium Ornstei n-Zernike relation, a closure relation, and a nonequilibrium potential and is solved in spherical harmonics. The distortion of the fluid structure due to shear is presented as the difference between the nonequilibrium and equ ilibrium pair distribution functions. From comparison of the results of the ory against results of equilibrium molecular dynamics with the nonequilibri um potential at low shear rates, it is concluded that, for a given nonequil ibrium potential, the theory is reasonably accurate, especially with the mo dified hypernetted chain closure. The equilibrium molecular-dynamics result s with the nonequilibrium potential are also compared against the results o f nonequilibrium molecular dynamics and suggest that the nonequilibrium pot ential used is not very accurate. In continuing work, a nonequilibrium pote ntial better suited to high shear rates [H. H. Can and B. C. Eu, Phys. Rev. A 46, 6344 (1992)] is being tested. [S1063-651X(99)05608-1].