  Water absorption in the metal-organic framework (MOF) material Cu-BTC, up to
a concentration of 3.5 H$_2$O per Cu ion, is studied via density functional
theory at the meta-GGA+U level. The stable arrangements of water molecules show
chains of hydrogen-bonded water molecules and a tendency to form closed cages
at high concentration. Water clusters are stabilized primarily by a combination
of water-water hydrogen bonding and Cu-water oxygen interactions. Stability is
further enhanced by van der Waals interactions, electric field enhancement of
water-water bonding, and hydrogen bonding of water to framework oxygens. We
hypothesize that the tendency to form such stable clusters explains the
particularly strong affinity of water to Cu-BTC and related MOFs with exposed
metal sites.
