I see that the definition of the stress-energy tensor refers to the flux of momentum across a surface but given the example of a collection of electrons confined to a box (volume in space), even with no macroscopic flow, the potential energy of the electrostatic repulsion between the electrons would surely contribute to the stress-energy of the system? A similar scenario with neutral particles would have less energy correct? But if the stress-energy tensor is defined in terms of flux, and there is no flow of mass in the box, then how is this potential energy accounted for?
All of the examples I have seen dealing with the stress-energy tensor discuss ideal fluids. I assume this model can actually be applied to any arbitrary configuration of matter, accounting for non-ideal characteristics such as molecular bonding and electrostatic potential energy?