We prove that the stress tensor conservation equation expressing the local equilibrium condition of a body results from the invariance of its partition function under canonical point transformations. From this result the expression of the stress tensor of a general atomistic system (with short range interactions) in terms of its microscopic degrees of freedom can be obtained. The derivation, which can be extended to encompass the quantum mechanical case, works in the canonical as well as the micro-canonical ensemble and is valid for systems endowed with arbitrary boundary conditions. As an interesting by-product of our general approach, we are able to positively answer the old question concerning the uniqueness of the stress tensor expression.
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