Parallel tempering Monte Carlo simulations of the water heptamer anion have been performed for temperatures ranging from 42 to 200 K. At low temperatures, a single peak near 250 meV in the electron binding energy distribution is obtained, while at high temperatures a second, weak peak near 450 meV is obtained, in good agreement with those observed experimentally. It is further confirmed that the high electron binding energies are due to hydrogen bonding networks with large net dipole moments and, in most cases, also containing a single double-acceptor monomer, while weak electron binding arises from configurations with smaller dipoles. (C) 2008 Elsevier B.V. All rights reserved.