To evaluate ∇[V ], one needs to take the derivative on each term of V expressed in Eq. 11, including ∇E ν (R) and ∇(ϵ • µ γν (R)). The first term is the gradient of the adiabatic electronic state energy, which can be obtained from the ab initio electronic structure calculation. The second term can be further expanded using Eq. 16 as