Temperature-dependent bond constraint theory (BCT) relies on counting the number of atomic degrees of freedom for a given topology of a glass network. It has been proven useful as a simplistic approach towards the prediction of glass properties. However, it breaks down at the inclusion of ionic bonds and is therefore presently unable to distinguish the effects of varying cationic species with predominantly ionic bonding. Here, we consider the treatment of modifier ions in the scope of BCT. Using the example of metaphosphate glasses with a broad range of modifier cation species, we find that the theory fails to predict the glass properties because of the specific contribution of each modifier species to the rigidity of the glass network.We therefore introduce the concept of constraint strength, which is a simplistic measure of how strongly the modifiers are bound to the surrounding oxygens through columbic forces
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