U=12⁢∫ℝ6ρ⁡(x,y,z)⁢ρ⁡(x′,y′,z′)⁢ⅆ⁡x⁢ⅆ⁡y⁢ⅆ⁡z⁢ⅆ⁡x′⁢ⅆ⁡y′⁢ⅆ⁡z′(x-x′)2+(y-y′)2+(z-z′)2.