We simulate α - Si21 –N2
: H that contains 6-atom boat-type rings and a – Si17–iNi
: H that contains 5-atom planar rings, where i = 0, 1 or 4. The simulations were carried out using the DFT-LDA approximation contained in the DMol code of MSI. We report calculations of impurity levels and relate these to the size of the gap and to the optical absorption curves for each cluster. A comparison is made between the two α – SiN clusters studied to analyze the effect of the ring topology.