THE BULK MODULUS OF COVALENT RANDOM NETWORKS

We re-examine the calculation of the bulk modulus of random networks w ith a variable mean coordination [r] that ranges from [r] = 4 down thr ough the phase transition from rigid to floppy that occurs at around [ r] = 2.4. In contrast to previous workers, we use random-network model s, rather than depleted diamond lattices, so our results are more rele vant to glasses, We find that the bulk modulus behaves in a very simil ar way to that found previously for depleted diamond lattices, with th e 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 transi tions between different local minima, especially in networks with a lo w mean coordination. We discuss and illustrate the nature of these met astable states and show that although there are differences in the loc al minimum energy associated with these states, the bulk modulus (curv ature around the minimum) is essentially independent of which minima t he system is in. We show that these nearly degenerate local minima are associated with different local confirmations of very short polymer c hains.