  By means of hybrid density functional theory we investigate the evolution of
the structural, electronic and magnetic properties of the colossal
magnetoresistance (CMR) parent compound LaMnO$_3$ under pressure. We predict a
transition from a low pressure antiferromagnetic (AFM) insulator to a high
pressure ferromagnetic (FM) transport half-metal (tHM), characterized by a
large spin polarization (~ 80-90 %). The FM-tHM transition is associated with a
progressive quenching of the cooperative Jahn-Teller (JT) distortions which
transform the $Pnma$ orthorhombic phase into a perfect cubic one (through a
mixed phase in which JT-distorted and regular MnO6 octahedra coexist), and with
a high-spin (S=2, m_Mn=3.7 mu_B) to low-spin (S=1, m_Mn=1.7 mu_B) magnetic
moment collapse. These results interpret the progression of the experimentally
observed non-Mott metalization process and open up the possibility of realizing
CMR behaviors in a stoichiometric manganite.
