  In this work we investigate the Fe$_3$O$_4$(001) surface/water interface by
combining several theoretical approaches, ranging from a hybrid functional
method (HSE06) to density-functional tight-binding (DFTB) to molecular
mechanics (MM). First, we assess the accuracy of the DFTB method to reproduce
correctly HSE06 results on structural details and energetics and available
experimental data for the adsorption of isolated water, dimers, trimers, etc.
up to a water monolayer. Secondly, we build two possible configurations of a
second and a third overlayer and perform molecular dynamics simulations with
DFTB, monitoring the water orientation, the H-bond network, and ordered water
structures formation. To make our models more realistic, we then build a 12-nm
thick water multilayer on top of the Fe$_3$O$_4$(001) surface slab model, which
we investigate through MM molecular dynamics. The water layers structuring,
revealed by the analysis of the atomic positions from a long MM-MD run for this
large MM model, extends up to about 6-7 {\AA} and nicely compares with that
observed for a water trilayer model. However, MM and DFTB MD simulations show
some discrepancy due to the poor description of the Fe--OH2 distance in MM that
calls for further work in the parametrization of the model.
