**The origin of the distinction between microscopic formulas for stress
and Cauchy stress**

YP Chen, EPL, 116, 34003 (2016).

DOI: 10.1209/0295-5075/116/34003

Stress is calculated routinely in atomistic simulations. The widely used microscopic stress formulas derived from classical or quantum mechanics, however, are distinct from the concept of Cauchy stress, i. e., the true mechanical stress. This work examines various atomistic formulations for local stress and their inconsistencies. Using standard mathematic theorems and the law of mechanics, we show that Cauchy stress results unambiguously from the definition of internal force density, thereby removing the long-standing confusion about the atomistic basis of the fundamental property of Cauchy stress and leading to a new atomistic formula for local stress The new atomistic stress formula has clear physical meaning and well-defined values, satisfies the conservation law, and is fully consistent with the concept of Cauchy stress. Copyright (C) EPLA, 2016

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