  Using first-principles calculations in combination with special quasirandom
structure and occupation control matrix methods, we study the magnetic ordering
and the effect of pressure on manganese sulfide polymorphs. At ambient
conditions, MnS is commonly observed in paramagnetic rock-salt structure, but
as temperature decreases at constant pressure it becomes antiferromagnetic. On
the other hand, at room temperature MnS has shown to undergo structural
transformations as pressure increases. Here, we show that our approach
involving the ordering/disordering of the local magnetic moments in addition to
the explicit dealing with the localization of the Mn $d$-electrons produces
energy band gaps and local magnetic moments in excellent agreement with those
observed experimentally, particularly for paramagnetic MnS. Finally, we focus
on how MnS evolves under pressure and from its enthalpy landscape we identify
at about 21~GPa, the structural transformation from rock-salt to orthorhombic
MnP-type. This structural transformation resembles closely experimental results
in which a new stable but unidentified MnS phase was previously reported.
