Thus, it can be obtained just by an arithmetic operation, and we do not have to solve linear equations iteratively. L (PR-MP2) with T P Q = 0 (and thus N P = 1) corresponds to the expression for the standard MP2, and λ ab P is obtained as the amplitude t ab P : Up to this point, the derivative of the PR-MP2 Lagrangian, L PR-MP2 = L HF +L (PR-MP2) , is stationary with respect to the changes in the amplitude but not in the MO coefficient: dL