A semi-empirical tight-binding method, GFN-XTB or GFN2-XTB, is employed in the QM2 region . The GFN2-XTB method accelerates the calculations of noncovalent interaction energies for complex molecular systems by accounting for anisotropic secondorder density fluctuation effects . Two schemes are employed, i.e., subtractive and additive , to account for interactions between the QM, QM2, and MM regions. Let the three regions in the receptor-ligand complex be defined as the high-level QM region (H), the low-level QM region (M), and the MM region (L). The QM and QM2 regions employ a subtractive scheme, where the QM-QM2 interaction is calculated by subtracting the contribution of the QM2 region from the total QM-QM2 interaction to ensure the accuracy of the calculations. In a subtractive scheme, three separate energy calculations are performed, i.e., QM calculation for the high layer, (E QM H ), QM2 calculation for the high and medium layer (E QM2 HM ), and a QM2 calculation for the high layer (E QM2 H ). The total energy for the subtractive scheme, E sub QM-QM2 , is given by: