def METHOD_NAME(self, geom: _GeomT) -> np.ndarray:
    """
    The derivative of the objective function.
    If not overridden, an approximation of the derivative is made
    using the 'central differences' method.
    This method is ignored if a non-gradient based algorithm is
    used when calling :meth:`.GeomOptimisationProblem.optimise`.
    """
    raise NotImplementedError