  Drawing on the theory of quantum mechanical stress, we introduce the stress
density in density functional theory. In analogy with the Chetty-Martin energy
density, the stress density provides a spatial resolution of the contributions
to the integrated macroscopic stress tensor. We give specific prescriptions for
a practical and efficient implementation in the plane wave ultrasoft
pseudopotential method within the local-density approximation. We demonstrate
the abilities of the stress density studying a set of representative test cases
from surface and interface physics. In perspective, the stress density emerges
as vastly more powerful and predictive than the integrated macroscopic stress.
