  Pair densities and associated correlation functions provide a critical tool
for introducing many-body correlations into a wide-range of effective theories.
Ab initio calculations show that two-nucleon pair-densities exhibit strong spin
and isospin dependence. However, such calculations are not available for all
nuclei of current interest. We therefore provide a simple model, which involves
combining the short and long separation distance behavior using a single
blending function, to accurately describe the two-nucleon correlations inherent
in existing ab initio calculations. We show that the salient features of the
correlation function arise from the features of the two-body short-range
nuclear interaction, and that the suppression of the pp and nn pair-densities
caused by the Pauli principle is important. Our procedure for obtaining
pair-density functions and correlation functions can be applied to heavy nuclei
which lack ab initio calculations.
