  We resum the ladder diagrams for the calculation of the energy density
$\cal{E}$ of a spin 1/2 fermion many-body system in terms of arbitrary vacuum
two-body scattering amplitudes. The partial-wave decomposition of the in-medium
two-body scattering amplitudes is developed, and the expression for calculating
$\cal{E}$ in a partial-wave amplitude expansion is also given. The case of
contact interactions is completely solved and is shown to provide renormalized
results, expressed directly in terms of scattering data parameters, within
cutoff regularization in a wide class of schemes. $S$- and $P$-wave
interactions are considered up to including the first three-terms in the
effective-range expansion, paying special attention to the parametric region
around the unitary limit.
