The modified Tersoff potential is used to calculate the ratio C/C₀ between the probability of vacancy formation at a particular site in the dislocation core and the corresponding quantity in ideal material. This ratio is given by the expression C/C₀ = exp (ΔS_f/k) exp (-ΔU_f/kT) in which ΔS_f and ΔU_f stand for the differences in formation entropy and energy, respectively. The results show that C/C₀ is significantly larger than one only for sites nearest to the dislocation line. It is also shown that the entropy factor is of minor importance.