To calculate and u A is the surface charge potential on QM atom A, i.e., u A , we solved ϕ tot (r) (total potential) and ϕ pc (r) (point charge potential) from Poisson's equation with the dielectric function ε(r) = 78.5 and ε ( r) = 1 respectively. Finite element interpolation was applied to obtain the total potential (ϕ tot A ) and point charge potential (ϕ pc A ) on QM atom A.