where q i , µ s,i and Θ i is the charge, the static dipole and the permanent quadrupole on site i, respectively, and V i is the electrostatic potential due to the rest of the permanent density of charge on site i, ∇ i V i is the gradient of the electrostatic potential and ∇ i ∇ i V i is the Hessian of the electrostatic potential V i .