  As is well known, the ground-state symmetry group of the water dimer switches
from its equilibrium $C_{s}$-character to $C_{2h}$-character as the distance
between the two oxygen atoms of the dimer decreases below $R_{\rm O-O}\sim 2.5$
\AA{}. For a range of $R_{\rm O-O}$ between 1 and 5 \AA{}, and for both
symmetries, we apply Partition Density Functional Theory (PDFT) to find the
unique monomer densities that sum to the correct dimer densities while
minimizing the sum of the monomer energies. We calculate the work involved in
deforming the isolated monomer densities and find that it is slightly larger
for the $C_s$ geometry for all $R_{\rm O-O}$. We discuss how the PDFT densities
and the corresponding partition potentials support the orbital-interaction
picture of hydrogen-bond formation.
