def FWHMeff2FWHMgeom(FWHMeff):
    """
    Convert FWHMeff to FWHMgeom.
    This conversion was calculated by Bo Xin and Zeljko Ivezic
    (and will be in an update on the LSE-40 and overview papers).

    Parameters
    ----------
    FWHMeff: float
        the single-gaussian equivalent FWHM value, appropriate for calcNeff, in arcseconds

    Returns
    -------
    float
        FWHM geom, the geometric FWHM value as measured from a typical PSF profile in arcseconds.
    """
    FWHMgeom = 0.822*FWHMeff + 0.052
    return FWHMgeom