Interplay of classical and quantum mechanics in the theory of charged-particle stopping

A quarter of a century ago the author stepped into Jens Oddershede's office and asked for support on a problem involving computation with atomic wave functions in connection with a new theoretical scheme to treat stopping of charged particles at intermediate speed. This visit resulted in two related publications, two joint papers and a number of follow-up studies by Jens and several others. In 1989 a Sanibel Symposium was devoted to aspects of the penetration of charged particles through matter, and since then, quite a few quantum chemists have joined the community of theoreticians dealing with particle penetration. Niels Bohr, a pioneer in both disciplines, emphasized the significance of classical vs. quantal arguments in particle penetration. Not the least in view of the complexity of ab initio computations in this area, such considerations keep being relevant. This note adds new points to an old discussion based on recent developments.