For the sake of saving memory and cpu time, in particular, for device
modelling, a new approximation to integrals over the Brillouin zone is
proposed, where the 3D integral is replaced with a sum of 1D integral
s along characteristic directions (e.g. symmetry lines) of the crystal
lattice. In contrast to parabolic-isotropic approximations, this meth
od takes into account the full non-parabolicity and approximates the w
arping of real bands. As a test, numerical results are given for the h
eavy- and light-hole density as function of Fermi energy in InGaAsP.