  Recent magnetic measurements on tetra-nickel molecular magnets
[Ni(hmp)(ROH)Cl]$_4$, where R=CH$_3$, CH$_2$CH$_3$, or
(CH$_2$)$_2$C(CH$_3$)$_3$ and hmp$^-$ is the monoanion of
2-hydroxymethylpyridine, revealed a strong exchange bias prior to the external
magnetic field reversal as well as anomalies in electron paramagnetic resonance
peaks at low temperatures. To understand the exchange bias and observed
anomalies, we calculate the electronic structure and magnetic properties for
the Ni$_4$ molecules with the three different ligands, employing
density-functional theory. Considering the optimized structure with possible
collinear spin configurations, we determine a total spin of the lowest-energy
state to be S=0, which does not agree with experiment. We also calculate
magnetic anisotropy barriers for all three types of Ni$_4$ molecules to be in
the range of 4-6 K.
