ABOUT APPROXIMATION OF CONVERGENCE SUPEROPERATORS IN QUANTUM PERTURBATION-THEORY

Perturbation methods are generally used for solving wave operator equa tions associated with the determination of effective Hamiltonians. In many cases the standard Rayleigh-Schrodinger and Brillouin-Wigner seri es either converge slowly or diverge. Therefore it is necessary to mod ify or to renormalize the standard wave equations. FOE that purpose de rivative and convergence superoperators within the Rayleigh-Schrodinge r and Brillouin-Wigner formalisms were introduced. A new efficient app roximation for convergence superoperators is investigated in this pape r. Its application to a model system of N non-interacting molecules sh ows that this approximation can overcome convergence difficulties.