  The effect of microscopic Mn cluster distribution on the Curie temperature
(Tc) is studied using density-functional calculations. We find that the
calculated Tc depends crucially on the microscopic cluster distribution, which
can explain the abnormally large variations in experimental Tc values from a
few K to well above room temperature. The partially dimerized Mn_2-Mn_1
distribution is found to give the highest Tc > 500 K, and in general, the
presence of the Mn_2 dimer has a tendency to enhance Tc. The lowest Tc values
close to zero are obtained for the Mn_4-Mn_1 and Mn_4-Mn_3 distributions.
