  We have developed a new method to derive the magnetic field distribution on
the surfaces of rotating magnetic white dwarfs from phase-resolved flux and
circular polarization spectra. An optimization code based on an evolutionary
strategy is used to fit synthetic Zeeman spectra for a variety of model
geometries described in the framework of a truncated multipole expansion. We
demonstrate that the code allows the reconstruction of relatively complex
fields using noise-added synthetic input spectra. As a first application, we
analyze flux and circular polarization spectra of the polar EF Eri in a low
state of accretion taken with FORS1 at the ESO VLT.
