  We develop a new implementation of the Gutzwiller approximation (GA) and
interface it with the local density approximation (LDA). This formulation
enables us to study complex $4f$ and $5f$ systems. We perform calculations of
praseodymium and $\alpha$-plutonium under pressure, which compare very well
with the experiments. Our study of praseodymium indicates that both structure
change and $f$-delocalization are important to obtain the correct phase diagram
and, in particular, the pressure-induced volume-collapse transition. Our
calculations of $\alpha$-plutonium indicate that, even though the $f$ electrons
are delocalized in this phase, the electron-correlations affect substantially
its electronic structure and thermodynamical properties.
