  An efficient physically motivated computational approach to multiband
superconductivity is introduced and applied to the study of the gap symmetry in
a heavy-fermion, UPd$_2$Al$_3$. Using realistic pairing potentials and accurate
energy bands that are computed within density functional theory,
self-consistent calculations demonstrate that the only accessible
superconducting gap with nodes exhibits d-wave symmetry in the $A_{1g}$
representation of the $D_{6h}$ point group. Our results suggest that in a
superconductor with gap nodes the prevailing gap symmetry is dictated by the
constraint that nodes must be as far as possible from high-density areas.
