  A new type of the single-molecule magnet [Mn_4 O_3 Cl_4 (O_2 CEt)_3(py)_3]
forms dimers. Recent magnetic hysteresis measurements on this single-molecular
magnet revealed interesting phenomena: an absence of quantum tunneling at zero
magnetic field and tunneling before magnetic field reversal. This is attributed
to a significant antiferromagnetic exchange interaction between different
monomers. To investigate this system, we calculate the electronic structure,
magnetic properties, intramolecular and intermolecular exchange interactions
using density-functional theory within the generalized-gradient approximation.
Our calculations agree with experiment. We also calculate vibrational infrared
absorption and Raman scattering intensities for the monomer which can be tested
experimentally.
