In this paper, two methods are described, by means of which it is possible to calculate the non rigid group of molecules consisting of a number of XH3 groups attached to a rigid framework. The first method is a combination of the wreath product formalism of Balasubramanian and modern computer algebra and the second method is a computational approach by using group theory package GAP. We apply these methods on 2,3,6,7,10,11-hexanitrotriphenylene (HNT) to compute its non-rigid group.