Density functional theory (DFT) methods, as implemented in the Amsterdam Density Functional Theory (ADF) program, were used to calculate the proton hyperfine coupling constants for [VO(H2O)(5)](2+). Qualitative agreement between the calculated and experimental proton hyperfine coupling constants for the axial water molecule in [VO(H2O)(5)](2+) was observed. For the equatorial water molecules, the proton hyperfine coupling constants depend on the orientation of the water molecule relative to the equatorial plane. DFT calculations revealed that the isotropic component of the proton hyperfine coupling constant for an equatorial water molecule has a trigonometric dependence on beta, where beta is the OVOH dihedral angle. The relative sizes of the isotropic hyperfine coupling constants for two protons on one water molecule can be used to determine the orientation of the water inolecule with respect to the equatorial plane. The computational results were compared with experimental single-crystal and powder ENDOR data from the literature .