where q A is the Blöchl point charge 27 on QM atom A obtained from AIMD calculations and u A is the surface charge potential on QM atom A. Briefly, in this procedure, the AIMD density, ρ AIMD (r), is converted a model density ρ model (r) that is constructed to have the same multipole moments, ∆Q lm = rr l Y lm (r) ρ AIMD (r) -ρ model (r) (37) and charge width