  The matrix Schroedinger equation with a selfadjoint matrix potential is
considered on the half line with the most general selfadjoint boundary
condition at the origin. When the matrix potential is integrable and has a
second moment, it is shown that the corresponding scattering matrix is
differentiable at zero energy. An explicit formula is provided for the
derivative of the scattering matrix at zero energy. The previously established
results when the potential has only the first moment are improved when the
second moment exists, by presenting the small-energy asymptotics for the
related Jost matrix, its inverse, and various other quantities relevantc to the
corresponding direct and inverse scattering problems.
