The ab initio molecular structures for several main group heptoxides are calculated using effective core potentials at the HF and DFT (B3LYP) levels. Particular attention is given to the X–O–X bond angle, as this structural parameter is a key feature for the study of both heptoxides and larger derivatives, such as polysilicate systems. The extent of the p–d interactions, which was found to be the main factor governing the magnitude of the X–O–X angle in transition metal heptoxides, does not play a significant role in the main group analogues. In the cyclic polysilicate systems (up to 12 silicon atoms), the Si–O–Si bond angle is found to vary from 132° in small rings to 150° in the less strained systems. The results are in agreement with the experimental data available and its accuracy competes with that of results from much heavier all-electron calculations.