ecollection s departures from plane-wave form near the atomic nuclei had very important con sequences for the energies of the states. Realizing that these departures were necessitated by the requirement that the valence electron wave functions be ortho- gonal to those of the cores, I thought of working with a non-orthogonal basis forme by orthogonalizing plane waves to the core states. To my surprise, even a single plane wave of this sort often gave a remarkably good quantitative approximation to the correct crystal wavefunction. So I started developing the approach system atically, and published an exposition of what is now known as the orthogonalized plane wave method. A fortunate opportunity presented itself immediately to test this in a full-scale band calculation: A G. Hill, with whom I was at that time sharing an apartment, had started some calculations with Seitz at Rochester on the band structure of beryllium, but had not finished them. Although primarily an experi mentalist, he was very interested in doing more work on this project, and he asked if i would be interested in collaborating with him. Since beryllium is divalent, and crystallizes in the hep structure with two atoms per primitive cell, its bands approximately fill two Brillouin zones instead of half a zone as in the alkali metals Thus calculation of its band structure and cohesive energy requires a method that is capable of handling states near the zone boundaries: an approximation using merely a parabolic band of constant effective mass would clearly be inadequate, and Shockley's'empty-lattice'test had just shown that sizable errors could occur in Slater's earlier scheme of fitting boundary conditions only at the centres of cell faces. So I proposed that we should combine OPW calculations for states at the zone boundary with Wigner-Seitz-Bardeen calculations for states near the bottom of the band. The project proved enormously laborious, but eventually we managed to get it finished, and it was on the whole quite successful In these same years at M.I.T., Marvin Chodorow, a graduate student under Slater, was working on the application of Slater's new APW method to copper feature of it was his construction of an empiricall n an this work. A noteworthy As his office adjoined mine, we had many conversations core potential for copper, significantly better than any that had been used before. Though the thesis was never published in full, use of the Chodorow potential has been revived in recent While this was going on, and afterward, I started to work on several applications of the opw method. One was an improved calculation of compressibility, etc, for lithium; another, moving towards my original goal of calculating interatomic force onstants, was an attempt, with H. B. Huntington, to evaluate the corrections to Fuchs's simple electrostatic theory of shear constants of simple metals. But such work went slowly, and after spending the next two years at Princeton and Missouri respectively, I became involved, as did everyone else, in war work for the period 1941-5. After the war, I became involved with other things, and it was with surprise that I learned several years later, in an encounter with Frank Herman at an A PS meeting in New York, that he was trying to do a thesis at Columbia University on the electronic bands of diamond, using the OPW method. I followed his work