Energy density and stress fields in quantum systems
There has been an enduring interest and controversy about whether or not one can define physically meaningful energy density and stress fields, $e(\bf{r})$ and $\sigma_{\alpha \beta}(\bf{r})$, since the two forms of the kinetic energy, $\frac{1}{2}|\nabla \Psi|^2$ and $-\frac{1}{2}\Psi\nabla^2 \Psi$, lead to different densities, and analogous issues arise for interactions. This paper considers the ground state of a system of many interacting particles in an external potential, and presents a resolution in steps. 1) For the kinetic energy all effects of exchange and correlation are shown to be unique functions of position $\bf{r}$; all issues of non-uniqueness involve only the density $n(\bf{r})$ and are equivalent to an effective single-particle problem with wavefunction $s(\bf{r}) = \sqrt{n(\bf{r})/N}$. 2) Interactions can be considered as potentials acting on particles or interaction fields, e.g., the Maxwell form in terms of electric fields. In each case, there is a mean field part that is a function of the density and a part due to correlation that is uniquely defined. 3) The final results follow from the nature of energy and stress. Because the energy determines the ground state itself through the variational principle, the kinetic energy must involve $-\frac{1}{2}s\nabla^2 s$ and interactions in terms of potentials. This leads to density functional theory interpreted as energy density $e(\bf{r})$ equilibrated to minimize fluctuations with the same chemical potential at all points $\bf{r}$. However, stress is related to forces, and the only acceptable expressions for the stress field involve the combination $\frac{1}{2}[s\nabla^2 s - |\nabla s|^2]$, and Coulomb interactions in terms of electric fields. Together these results lead to well-defined formulations of energy density and stress fields that are physically motivated and based on a clear set of arguments.