Systematic construction of efficient many‐body perturbation series

A new procedure for the splitting of many‐body Hamiltonians into ‘‘free’’ and ‘‘interaction’’ parts is proposed which leads to a rapidly converging perturbation expansion. The efficiency of this method is shown for the case of small molecules: Already first and second order perturbations turn out to produce very reasonable results even for excited states; higher terms rapidly converge to zero. An important point to notice is that our method allows for a priori estimates of the convergence (or divergence) behavior.