  The interplay between groups of water molecules and single-atom contacts, as
reflected in the electrical conductances and mechanical forces of copper atomic
junctions, is explored by means of first-principles theory and semi-empirical
calculations. We study the influence of the atomic geometries of copper
electrodes with pyramidal and non-crystalline structures in the presence and
absence of water on the conductance profiles as the electrodes approach each
other. It is shown that the atomic arrangements of nano-contacts have crucial
effects on the formation of plateaus and the conductance values. Groups of
hydrogen bonded water molecules bridge the junction electrodes before a direct
Cu-Cu contact between the electrodes is made. However, the bridging of the two
copper electrodes by a single H$_{2}$O molecule only occurs in the junctions
with pyramidal electrodes. Our findings reveal that the presence of H$_{2}$O
molecules modifies strongly the conductance profile of these junctions. In the
absence of water molecules, the pyramidal junctions exhibit continuous
transitions between integer conductance plateaus, while in the presence of
H$_{2}$O molecules, these junctions show abrupt jump to contact behavior and no
well-defined conductance plateaus. By contrast, in the absence of H$_{2}$O
molecules, the non-crystalline junctions display jump to contact behavior and
no well-defined plateaus, while in the presence of H$_{2}$O molecules they
exhibit a jump to contact and abrupt transitions between fractional and integer
plateaus.
