def sub_chempots(gamma_dict, chempots):
    """
    Uses dot product of numpy array to sub chemical potentials
        into the surface grand potential. This is much faster
        than using the subs function in sympy.
    Args:
        gamma_dict (dict): Surface grand potential equation
            as a coefficient dictionary
        chempots (dict): Dictionary assigning each chemical
            potential (key) in gamma a value
    Returns:
        Surface energy as a float
    """

    coeffs = [gamma_dict[k] for k in gamma_dict.keys()]
    chempot_vals = []
    for k in gamma_dict.keys():
        if k not in chempots.keys():
            chempot_vals.append(k)
        elif k == 1:
            chempot_vals.append(1)
        else:
            chempot_vals.append(chempots[k])

    return np.dot(coeffs, chempot_vals)