Virial stress

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Virial stress is a measure of mechanical stress on an atomic scale. It is given by

\tau_{ij} = \frac{1}{\Omega} \sum_{k \in \Omega} \left(-m^{(k)} (u_i^{(k)}- \bar{u}_i) (u_j^{(k)}- \bar{u}_j) + \frac{1}{2} \sum_{\ell \in \Omega} ( x_i^{(\ell)} - x_i^{(k)}) f_j^{(k\ell)}\right)

where

  • k and \ell are atoms in the domain,
  • \Omega is the volume of the domain,
  • m^{(k)} is the mass of atom k,
  • u_i^{(k)} is the ith component of the velocity of atom k,
  • \bar{u}_j is the jth component of the average velocity of atoms in the volume,
  • x_i^{(k)} is the ith component of the position of atom k, and
  • f_i^{(k\ell)} is the ith component of the force applied on atom k by atom \ell.

At zero kelvin, all velocities are zero so we have

\tau_{ij} = \frac{1}{2\Omega} \sum_{k,\ell \in \Omega} ( x_i^{(\ell)} - x_i^{(k)}) f_j^{(k\ell)}.

This can be thought of as follows. The τ11 component of stress is the force in the x1-direction divided by the area of a plane perpendicular to that direction. Consider two adjacent volumes separated by such a plane. The 11-component of stress on that interface is the sum of all pairwise forces between atoms on the two sides.

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