  The isotope masses and relative abundances for each element are fundamental
chemical knowledge. Computing the isotope masses of a compound and their
relative abundances is an important and difficult analytical chemistry problem.
We demonstrate that this problem is equivalent to sorting
$Y=X_1+X_2+\cdots+X_m$. We introduce a novel, practically efficient method for
computing the top values in $Y$. then demonstrate the applicability of this
method by computing the most abundant isotope masses (and their abundances)
from compounds of nontrivial size.
