def timestep(units='metal'):
    """
    Returns the default timestep value for a LAMMPS unit style option.
    
    Parameters
    ----------
    units : str, optional
        The LAMMPS unit option.  Default value is 'metal'
    
    Returns
    -------
    float
        The default timestep value.
    """
    if units == 'lj':
        return 0.005
    elif units == 'real':
        return 1.0
    elif units == 'metal':
        return 0.001
    elif units == 'si':
        return 1e-08
    elif units == 'cgs':
        return 1e-08
    elif units == 'electron':
        return 0.001
    elif units == 'micro':
        return 2.0
    elif units == 'nano':
        return 0.00045
    else:
        raise ValueError('units ' + units + ' not supported')
