  The ground state and excited state properties of the perovskite LaMnO$_3$ are
calculated based on the generalized-gradient-corrected relativistic
full-potential method. The electronic structure, magnetism and energetics of
various spin configurations for LaMnO$_3$ in the ideal cubic perovskite
structure and the experimentally observed distorted orthorhombic structure are
obtained. The excited state properties such as the optical, magneto-optical,
x-ray photoemission (XPS), bremsstrahlung isochromat (BIS), x-ray absorption
near edge structure (XANES) spectra are calculated and found to be in excellent
agreement with available experimental results. The present results suggest that
the correlation effect is not significant in LaMnO$_3$ and the presence of
ferromagnetic coupling within the $ab$ plane as well as the antiferromagnetic
coupling perpendicular to this plane can be explained through the itinerant
band picture. As against earlier expectations, our calculations show that the
Mn 3$d$ $t_{2g}$ as well as the $e_{g}$ electrons are present in the whole
valence band region. In particular significantly large amounts of $t_{2g}$
electrons are present in combination with the $e_{g}$ electrons at the top of
the valence band against the common expectation of presence of only pure
$e_{g}$ electrons. We have calculated the hyperfine field parameters for the
A-type antiferromagnetic and the ferromagnetic phases of LaMnO$_3$ and compared
the findings with the available experimental results. The role of the
orthorhombic distortion on electronic structure, magnetism and optical
anisotropy are analyzed.
