def _vol_for_cv_type(inp):
        if not HAS_SIMTK_UNIT and inp == 'simtk':
            pytest.skip()

        func = {
            'float': lambda s: 1.0,
            'array': lambda s: np.array([1.0, 2.0]),
            'array1': lambda s: np.array([1.0]),
            'simtk': None
        }[inp]
        if func is None:  # only if inp is 'simtk'
            func = lambda s: 1.0 * unit.nanometers

        cv = paths.FunctionCV('cv', func)
        volume = paths.CVDefinedVolume(cv, 0.0, 1.5)
        return volume