  The lattice distortion parameter $\delta \equiv c/a-\sqrt{8/3}$ has been
calculated as a function of molar volume for the hcp phases of He, Ar, Kr and
Xe. Results from both semi-empirical potentials and density functional theory
are presented. Our study shows that $\delta$ is negative for helium in the
entire pressure range. For Ar, Kr and Xe, however, $\delta$ changes sign from
negative to positive as the pressure increases, growing rapidly in magnitude at
higher pressures.
