The adiabatic electron detachment energy for (H2O...NH3)(-) has been found to be 109 cm(-1) at the coupled-cluster level of theory with single, double, and noninteractive triple excitations (CCSD(T)), to be compared with the recent experimental result of 123-129 cm(-1) obtained by Abdoul-Carime et al. [Z. Phys. D 40, 55 (1997)]. The stationary points on the potential energy surface of the neutral and anionic dimer have been determined at the second-order Moller-Plesset level of theory. Our results indicate that the second-order dispersion interaction between the loosely bound electron and electrons of the neutral dimer is as important as the electrostatic electron-dipole stabilization. The higher-order electron correlation corrections are also very important and the CCSD(T) electron binding energy is approximately four times larger than the Koopmans theorem estimation. In addition, the hydrogen bond in H2O...NH3 is susceptible to a deformation upon attachment of an electron. This deformation enhances both the electrostatic and dispersion components of the electron binding energy. The calculated Franck-Condon factors indicate that neutral dimers formed in electron photodetachment experiments may be vibrationally excited in both soft intermolecular and stiff intramolecular modes. The theoretical photoelectron spectrum based on the calculated Franck-Condon factors is reported.