For each structure, we run 200,000 MC pivot moves for which an hydrogen H of the framework is chosen randomly. An example of some of resulting silanol nests after optimization in silicalite-1 and chabazite are presented in Figure . We notice that three silanol groups are involved in a trimer of hydrogen bonds characterized by an imperfect O-H-O alignment, resulting from the applied steric constraints. These silanol nest geometric structures are in very good agreement with classical studies of Trzpit and Cailliez, the DFT calculations of Sokol et al. as well as those obtained from ab initio cluster calculations.