Selected structural parameters for the mono-substituted Keggin, [M(H2O)PW11O39] q-, and Wells-Dawson, [M(H2O)P2W17O61] q-, anions were examined against experimental crystallographic structures reported in Figure . Molecular geometries were optimized using several generalized gradient approximation (GGA), hybrid, and long-range-corrected GGA (LC-GGA). Hybrid functionals with increasing contributions of Hartree-Fock (HF) exchange (0 % PBE , 15 % B3LYP*, 20 % B3LYP, 25 % PBE0, and 50 % BH&H) were employed to examine structural sensitivity of mono-substituted polytungstate as a function of HF exchange. To evaluate the accuracy of functionals on the structural optimization of [M(H2O)PW11O39] q-anions; mean unassigned error (MUE) and mean signed error (MSE) were used to compare with experimental crystallographic structures. There are four types of oxygen atoms which will be examined, displayed shown in Figure . anion referenced against the crystallographic structure taken from Cavaleiro et al. The mean unsigned error (MUE) and mean signed error (MSE) account for all four types of metal-oxygen interactions. All bond distances are reported in Å. Analysis of the basis set was restricted to the GGA-PBE functional and the effect of the XC-functional was confined to TZP basis set.