  We present LDA+$U_{sc}$ calculations of high-spin (HS) and low-spin (LS)
states in ferropericlase (fp) with an iron concentration of 18.75$\%$. The
Hubbard parameter $U$ is determined self-consistently with structures optimized
at arbitrary pressures. We confirm a strong dependence of $U$ on the pressure
and spin state. Static calculations confirm that the antiferromagnetic
configuration is more stable than the ferromagnetic one in the HS state,
consistent with low-temperature measurements. Phonon calculations guarantee the
dynamical stability of HS and LS states throughout the pressure range of the
Earth mantle. Compression curves for HS and LS states agree well with
experiments. Using a non-ideal mixing model for the HS to LS states solid
solution, we obtain a crossover starting at $\sim$45 GPa at room temperature
and considerably broader than previous results. The spin-crossover phase
diagram is calculated, including vibrational, magnetic, electronic, and
non-ideal HS-LS entropic contributions. Our results suggest the mixed-spin
state predominates in fp in most of the lower mantle.
