The total force F k on particle k is the sum of all the forces from the other particles j in the system F k = ∑ ∑ j = 1 N F j k where F jk is the force applied by particle j on particle k. Hence, the virial can be written − − 1 2 ∑ ∑ k = 1 N F k ⋅ ⋅ r k = − − 1 2 ∑ ∑ k = 1 N ∑ ∑ j = 1 N F j k ⋅ ⋅ r k .