Probably the most widely used theory for quantitative prediction in condensed matter physics is the density functional theory (DFT), originally developed by Hohenberg and Kohn . This theory has the advantage that it does not require adjustable parameters from experiments, in principle only fundamental constants as the speed of light in vacuum, Planck's constant, electron charge, etc. are taken from the experiment. DFT allows to calculate the ground-state properties of materials: total energy, ground-state lattice constants, electronic structure etc. In magnetism it appeared to be particularly usefull for calculation of spin-dependent band structure in transition metals, such as Co.
Density functional theory promotes the electron density
instead of the many particle wave function as the key variable, on which the calculation of all other observables can be based.