Independent time approximation for dynamically interacting multi-electron systems

We propose an independent time approximation (ITA), defined by simplifying the chronological operator, namely, T≃T_unc, where T_unc is the uncorrelated limit of the chronological operator T. The concept of the time correlation is analogous to spatial electron correlation. In both cases `correlation' is the difference between an exact result and an uncorrelated limit. In the uncorrelated ITA the time correlation operator T_cor = T-T_unc is neglected and causality between electrons is lost. In second-order calculations of a two-electron transition the ITA reduces computer time by over two orders of magnitude. Greater savings are expected when higher orders or more electrons are included.