  We present an ab-initio study of the phase transition cd->beta-tin in Si and
Ge under hydrostatic and non-hydrostatic pressure. For this purpose we have
developed a new method to calculate the influence of non-hydrostatic pressure
components not only on the transition pressure but also on the enthalpy
barriers between the phases. We find good agreement with available experimental
and other theoretical data. The calculations have been performed using the
plane-wave pseudopotential approach to the density-functional theory within the
local-density and the generalized-gradient approximation implemented in VASP.
