Virial stress is a measure of mechanical stress on an atomic scale. It is given by
where
- and are atoms in the domain,
- is the volume of the domain,
- is the mass of atom k,
- is the ith component of the velocity of atom k,
- is the jth component of the average velocity of atoms in the volume,
- is the ith component of the position of atom k, and
- is the ith component of the force applied on atom by atom .
At zero kelvin, all velocities are zero so we have
- .
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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