  We derive the exact Foldy-Wouthuysen transformation for Dirac fermions in a
time independent external electromagnetic field in the basis of the Ritus
eigenfunctions, namely the eigenfunctions of the operator $(\gamma \cdot
\Pi)^2$, with $\Pi^\mu = p^\mu - e A^\mu$. In this basis, the transformation
acquires a free form involving the dynamical quantum numbers induced by the
field.
