def get_expected_occupancy(grid_spacing):
    """Return number of expected probes in a rectangular grid element.

    Calculation is based on a reference simulation of water and probe mixture
    (see :ref:`mixture`).

    """
    gs3 = grid_spacing * grid_spacing * grid_spacing
    n_probes = 343                # of isopropanols
    #nW = 6860                    # of waters
    ave_vol = 240934.57           # from a 10 ns reference simulation
    #return (nW*gs3/ave_vol, nI*gs3/ave_vol)
    return n_probes * gs3 / ave_vol