The IGM method defines a function 𝛿𝑔(𝐫) = 𝑔 IGM (𝐫) -𝑔(𝐫) , where g represents the gradient norm of the promolecular density, and g IGM represents the gradient norm of IGM type. It is defined in such a way that the density gradients of free-state atoms do not cancel each other out due to different signs when summed.