The bulk modulus of covalent random networks

We re-examine the calculation of the bulk modulus of random networks with a variable mean coordination (r) that ranges from (r) = 4 down through the phase transition from rigid to floppy that occurs at around (r) = 2.4. In contrast to previous workers, we use random-network models, rather than depleted diamond lattices, so our results are more relevant to glasses. We find that the bulk modulus behaves in a very similar way to that found previously for depleted diamond lattices, with the bulk modulus going to zero at around (r) = 2.4 with an exponent of 1.4. In the course of this study we came across many examples of transitions between different local minima, especially in networks with a low mean coordination. We discuss and illustrate the nature of these metastable states and show that although there are differences in the local minimum energy associated with these states, the bulk modulus (curvature around the minimum) is essentially independent of which minima the system is in. We show that these nearly degenerate local minima are associated with different local confirmations of very short polymer chains.