After introduction of silanol defects as described above, we performed a local optimization of the defect structure using the force field described in the next section, by allowing each H atom to freely rotate around the Si-O bond while the rest of the framework remains frozen. We used the RASPA simulation package in which we implemented the version of the pivot algorithm proposed by Steinhauser. For each oxygen atom in a triplet Si-O-H, we define the unit vector #» n pointing from Si to O atom: