The isotopic fractionation factor in the monohydrated gas-phase cluster Cl- (H2O), i.e., the equilibrium constant for Cl -(H2O)(g) + D2O(g) ⇄Cl- (D2O)(g) + H2O(g), is used to test models used for solution-phase simulations and to test semiempirical and ab initio molecular orbital theory for their detailed structural and vibrational predictions for both the solute-solvent bond properties and the interaction-induced intramolecular changes in water itself. The isotope effect is studied at a consistent level of vibration-rotation theory, i.e., the harmonic-oscillator- rigid-rotator approximation, using four different kinds of approach, namely extended-basis-set ab initio electronic structure calculations, both (i) with and (ii) without electron correlation, (iii) semiempirical molecular orbital theory at the level of neglect of diatomic differential overlap, and (iv) analytic force fields based on site-site interactions. The calculations of type (i) show good convergence and are compared both to experiment, which presumably tests the harmonic approximation, and to the results of efforts (ii), (iii), and (iv), which presumably tests the structural and vibrational properties predicted by these more approximate approaches. We find significant effects of anharmonicity and electron correlation; semiempirical molecular orbital theory does remarkably well; and there is a wide variation in predictions among the 25 analytic force fields tested. Finally we combine the well converged ab initio results for the properties of the well, the vibrational red and blue shifts, and the isotope effect on the equilibrium constant with the previous ab initio calculations of Dacre for the repulsive interactions to obtain a new interaction potential for chloride ion with nonrigid water that also predicts a more accurate enthalpy of complexation and binding energy than do the chloride-water potentials available in the literature.