Ab initio molecular geometries and vibrational frequencies of various isolated vanadate species NO,"-, HVO;-, H2V0;, and V,O;-) were calculated using different pseudopotentials. The relative merits of these were assessed by comparing the calculated molecular parameters with the corresponding values obtained from calculations at all-electron levels and, whenever available, from X-ray studies for the salts. The calculations were extended to higher oligomers (V,O:;, V40:;, and V40t; ) using the pseudopotential whose basis functions are (10s5p5d)/[2sl p l d ] (55/5/5) on vanadium and ( 4 s 4 p ) / [ 2 s 2 p ] (31/31) on oxygen, which yielded the best compromise between accuracy and computational effort. The results indicate a linear centrosymmetric geometry for the isolated V,O;anion. The higher oligomers have less than 180" V-0-V angles, except the noncyclic tetraoligomer which has a linear central V-0-V angle (180"). The cyclic V40f; species presents a planar structure with all the vanadium and bridging oxygen atoms in the same plane. This structure was already reported for the [(CH,)CNH,][ V40,, I salt. The results suggest a lower stability of the linear V40!; species, in agreement with previous reports. 0 1996 by John Wiley & Sons, Inc. by its biological impact.'-3 In particular, it has been demonstrated that vanadium salts display insulin-like activity and may be used in the treatment of diabetes., Most of the biological importance of vanadium is associated with the +5 oxidation state (vanadate) and is due to a similarity between the vanadate and the phosphate lntroduction hile vanadium is a trace element, toxic at high concentration levels, it is well known vv