  The dimeric form of the single-molecule magnet
[Mn$_4$O$_3$Cl$_4$(O$_2$CEt)$_3$(py)$_3$]$_2$ recently revealed interesting
phenomena: no quantum tunneling at zero field and tunneling before magnetic
field reversal. This is attributed to substantial antiferromagnetic exchange
interaction between different monomers. The intermolecular exchange
interaction, electronic structure and magnetic properties of this molecular
magnet are calculated using density-functional theory within
generalized-gradient approximation. Calculations are in good agreement with
experiment.
