“…Internal rotation in molecules (one of the types of large amplitude motion (LAM)) continues to attract the attention of researchers today [1][2][3][4][5][6][7]. This is due to the important role that internal rotation plays in determining the thermodynamic properties of molecules [1], as well as in the formation of the fine structure of rotational and vibrational-rotational absorption bands [8][9][10]. Although early works addressed only molecules with a single internal top [11][12][13][14][15], research soon expanded to molecules with two or more internal tops.…”