Binding free energy (ΔG) of RBD-antibody or ACE2-RBD complexes was calculated by MM/GBSA method using GB OBC model(igb = 5)with a salt concentration of 150 mM. 750 snapshots extracted evenly from 50-200ns trajectories were used for binding free energy calculation. In this study, the internal and external dielectric constants were set to be 1.0 and 78.5 separately. The free energy decomposition analysis was carried out using an internal program with idecomp = 1.