  Deuteron correlation energy (DCE) of the valence proton-neutron subsystem is
evaluated by utilizing a simple three-body model. We focus on the $^6$Li and
$^{18}$F nuclei assuming the doubly-closed core and the valence proton and
neutron. Two interaction models, schematic density-dependent contact (SDDC) and
Minnesota potentials, are utilized to describe the proton-neutron interaction.
Evaluating DCE, we conclude that the proton-neutron binding in $^6$Li can be
stronger than its counterpart of a deuteron in vacuum. On the other hand, in
$^{18}$F, the energetic correlation is remarkably weak, and does not favor the
bound, deuteron-like configuration. This significant difference between two
systems can be understood from a competition between the proton-neutron kinetic
and pairing energies, which are sensitive to the spatial extension of the wave
function. This result indicates a remarkable dependence of the deuteron
correlation to its environment and the valence orbits.
