The behaviour of electrons is described by the quantum electrodynamics, which is currently the most accurate, most precisely tested physical theory. Richard Feynman, who pioneered it, writes in his famous book, “QED”:

Richard Feynman did not wish to become more specific about what tremendous and a few meant, presumably for good reasons. The truth, even several decades after the publication of Feynman’s famous book is that – in the above context – two electrons already constitute a tremendous number while three or more deserve the attribute: horrendously tremendous… In fact there is no theoretical model in existence that could accurately and reliably predict the motion of two or more bound electrons under some external influence. Given the limited computing power available, simplified models must replace the Schrödinger equation to describe microscopic phenomena of practical significance, which almost always involve the motion of several or more electrons. These models can’t be developed without experimental means of testing their prediction. One of the main missions of experimental attosecond physics is to provide these means and thereby help develop models that can accurately describe the behaviour of complex, many-electron systems, as they appear in nature, with today’s computing powers.