  Using the full potential linearized augmented plane wave plus local orbitals
method we determine ab-initio the lattice parameters of tetragonally distorted
Co2TiSn in the L21 structure. The tetragonal lattice parameter c is determined
as a function of the lattice parameter a by energy minimization. The change in
total energy is found to be only a few $k_B T$ with respect to room
temperature. The spin polarizations as well as the magnetizations are stable
against small lattice distortions. It is shown, that the volume is not constant
upon distortion and that the volume change is related with significant changes
in the magnetization and the gap energy.
