  We quantify the total, quantum, and classical correlations with entropic
measures, and quantitatively compare these correlations in a quantum system, as
exemplified by a Heisenberg dimer which is subjected to the change of
environmental parameters: temperature and nonuniform external field. Our
results show that the quantum correlation may exceed the classical correlation
at some nonzero temperatures, though the former is rather fragile than the
later under thermal fluctuation. The effect of the external field to the
classical correlation is quite different from the quantum correlation.
