Electronic structure and half-metallic transport in the<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">La</mml:mi></mml:mrow><mml:mrow><mml:mn>1</mml:mn><mml:mi mathvariant="normal">−</mml:mi><mml:mi mathvariant="italic">x</mml:mi></mml:mrow></mml:msub></mml:mrow></mml:math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">Ca</mml:mi></mml:mrow><mml:mrow><mml:mi mathvariant="italic">x</mml:mi></mml:mrow></mml:msub></mml:mrow></mml:math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">MnO</mml:mi></mml:mrow><mml:mrow><mml:mn>3</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>system

Possible origins of ``colossal magnetoresistance'' (CMR) behavior in the ${\mathrm{La}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ca}}_{\mathit{x}}$${\mathrm{MnO}}_{3}$ system are studied using the local spin-density method. These calculations allow the quantification of the effects of Mn d--O p hybridization that have been largely neglected in previously published work. As regards the end-point compounds ${\mathrm{CaMnO}}_{3}$ and ${\mathrm{LaMnO}}_{3}$, the very different structural and magnetic symmetries of their ground states are predicted correctly. The distortion from the cubic perovskite structure of the ${\mathrm{LaMnO}}_{3}$ lattice is necessary to produce an antiferromagnetic insulating ground state. The distortion also strengthens the Mn magnetic moments. Application to ferromagnetic and constrained ferrimagnetic phases of ${\mathrm{La}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ca}}_{\mathit{x}}$${\mathrm{MnO}}_{3}$ in the CMR regime x\ensuremath{\approxeq}1/4--1/3 suggests, as observed, that magnetic coupling switches from antiferromagnetic to ferromagnetic. Hybridization between Mn d states and O p states is found to be strongly spin dependent, because the majority Mn d bands overlap the O p bands while the minority Mn d bands are separated by a gap from the O p bands. Both ferromagnetic and ferrimagnetic orderings are obtained and compared. We identify strong local environment effects arising from neighboring cation charge differences (${\mathrm{La}}^{3+}$ or ${\mathrm{Ca}}^{2+}$) that suggest localization of the low density of minority carriers, leading to effective half-metallic ferromagnetism in the CMR regime. This behavior supports in some respects the popular ``double exchange'' picture of Zener but indicates the Mn d--O p hybridization is much too strong to be considered perturbatively. Half-metallic character promotes the possibility of very large magnetoresistance, and may well be an essential ingredient in the CMR effect.