Atomic and molecular representations of molecular hydrodynamic variables

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Abstract
Formulae connecting the atomic and molecular representations of the microscopic momentum density and stress (pressure) tensor are used to relate corresponding equilibrium time correlation functions and non-equilibrium response functions. It is shown that, while the atomic and molecular stresses are essentially equivalent for calculating transport coefficients, and the appropriate correlation functions become similar as t → ∞, a residual difference between these functions, associated with internal molecular motion and reorientation, may persist at intermediate and long times. It is also demonstrated that the low frequency, atomic and molecular stress responses to a homogeneous oscillatory shear are essentially equivalent. The problem of formulating consistent hydrodynamic theories in the atomic and molecular representations is discussed.