Virial theorem and pressure calculations in the GGA

It is shown that using a generalized gradient approximation (GGA) functional describing exchange and correlation (xc) effects in electronic structure calculations implies additional xc terms in the virial theorem and the pressure tensor. These new expressions generalize the forms well known in the local density approximation and are useful for checking the numerical precision of self-consistent field calculations for atoms and solids, and for accurate pressure computations in heavy elements. Numerical examples are given.