Return the centre of mass (COM).

    Parameters
    ----------
    elements : numpy.ndarray
        An array of all elements (type: str) in a molecule.

    coordinates : numpy.ndarray
        An array containing molecule's coordinates.

    Returns
    -------
    numpy.ndarray
        An 1d array with coordinates of the centre of mass including elements'
        masses.

def center_of_mass(elements, coordinates):
    """
    Return the centre of mass (COM).

    Parameters
    ----------
    elements : numpy.ndarray
        An array of all elements (type: str) in a molecule.

    coordinates : numpy.ndarray
        An array containing molecule's coordinates.

    Returns
    -------
    numpy.ndarray
        An 1d array with coordinates of the centre of mass including elements'
        masses.

    """
    mass = molecular_weight(elements)
    mass_array = np.array([[atomic_mass[i.upper()]] * 3 for i in elements])
    mass_coordinates = coordinates * mass_array
    return (np.sum(mass_coordinates, axis=0) / np.array([mass, mass, mass]))