  We study the composition dependent evolution of geometric and magnetic
structures of MnO clusters within density functional theory. A systematic and
extensive search through the potential energy surface is performed to identify
the correct ground state, and significant isomers. We find that the magnetic
structures in these MnO clusters are complex, which has been explained using
the intrinsic electronic structure of the cluster, and analyzed using model
Hamiltonian with parameters obtained from maximally localized Wannier
functions. The calculated vertical displacement energies of off-stoichiometric
MnO clusters compare well with the recent experimental results. Interestingly,
the charged state of the cluster strongly influences the geometry and the
magnetic structure of the cluster, which are very different from the
corresponding neutral counterpart. Further, the importance of electron
correlation in describing simple Mn-dimer and MnO clusters has been discussed
within Hubbard model and hybrid exchange-correlation functional.
