In this thesis, we have investigated the structural, electronic, and magnetic properties of purernLaMnO3 from first-principles using the quantum-ESPRESSO open-source code. Unlike previousrntheoretical works, most of which implemented experimental structure, we have performed fullrnstructural relaxations for both the cubic and orthorhombic structures within the GGA and LDA+Urnapproaches. Our simulated structural parameters are in very good agreement with experimentalrnvalues. We have used these theoretical geometries in all the calculations. Consistent with therngeneral theory of the CMR manganites but not reported in earlier theoretical works, the bandrnstructure and DOS calculations for the cubic ferroelectric phase predict half-metallicity. We havernalso examined the phonon-dispersion and the corresponding density of states for the theoreticallyrnoptimized cubic ferromagnetic LaMnO3 from which we are able to observe two strong crystalrninstabilities. The phonon dispersion is calculated along the high-symmetry axes throughout thernBrillouin zone. The densities of states calculated for the orthorhombic structure with differentrnmagnetic orderings strongly suggest the need for structural relaxation in order to get thernexperimentally observed insulating ground state for LaMnO3. Both GGA and LDA+U calculationsrnfor the fully relaxed orthorhombic A-type antiferromagnetic structure reproduce the insulatingrncharacter. However, the value of the insulating gap obtained within the LDA+U case is muchrncloser to experiment than that of GGA, which in turn means that a better description of LaMnO3 isrnpossible with the inclusion of the on-site coulomb interaction.