def METHOD_NAME(self) -> np.ndarray:
    """Get the column of the properties array containing formal charges.
    If there is no property with the name 'formal charge', this raises a ValueError.
    Returns
    -------
    a numpy array containing the requested column of the properties array.  This
    is a 1D array of length num_atoms.
    """
    return self.get_property('formal charge')