In a common special case, the potential energy V between two particles is proportional to a power n of their distance r ij V j k = α α r j k n , where the coefficient α and the exponent n are constants. In such cases, the virial is given by the equation − − 1 2 ∑ ∑ k = 1 N F k ⋅ ⋅ r k = 1 2 ∑ ∑ k = 1 N ∑ ∑ j < k d V j k d r j k r j k = 1 2 ∑ ∑ k = 1 N ∑ ∑ j < k n α α r j k n − − 1 r j k = 1 2 ∑ ∑ k = 1 N ∑ ∑ j < k n V j k = n 2 V TOT where V TOT is the total potential energy of the system V TOT = ∑ ∑ k = 1 N ∑ ∑ j < k V j k .