Ab initio treatment of (H2O)(2)(-) and (H2O)(6)(-)

MP2 level investigations of (H2O)(6)(-) are reported which have been carrie d out to assign its vertical electron detachment (VDE) spectrum. Extended b asis sets including a floating center are employed which yield an accuracy of a few 0.01 eV for the VDE of (H2O)(2)(-) and (H2O)(3)(-) used as test ca ses. Results for (H2O)(6)(-) differ considerably from previous treatments a nd lead to a new assignment of the VDE spectrum. It also turns out that (H2 O)(6)(-) is probably adiabatically unstable since total energies are margin ally higher than for the ground state of the neutral species in the most st able geometries.