recollections 71 associated with the polarization of the conduction electrons, i. e with their Pauli paramagnetism. The magnitude of the effect should be proportional to the product of the spin susceptibility and the average s character of the Fermi-surface electrons Since crude ways of estimating the latter could be significantly in error, he asked me to collaborate with them in working out some reasonably good estimates metals whose band structures had been calculated. Some of the comparisons of theory and experiment were gratifying, others could more aptly be described as intriguing; such comparisons have subsequently become the object of a sizable literature Despite the advances associated with the APW and OPW methods of calculating band energies and wavefunctions, such calculations were still quite laborious, and there was a great need for something simpler that could be used to make calculations of such things as phonon frequencies, ete. In the 1930s, the Russian physical hemist, Hellmann, had noted (as I too later noted in developing the OPW method) that the rapid oscillations of a valence-electron wavefunction near the nucleus of an atom are necessitated by the requirement of orthogonality to the cores. He proposed replacing the orthogonality requirement by a positive effective potential in the core region and gave a rough derivation of such a positive potential on the basis of the Fermi-Thomas model of the atom. He then proposed that one could choose this repulsive potential empirically, rather than relying on first-principles alculation of it: namely, one could choose a few parameters describing the repulsive potential in such a way that a nodeless eigenfunction in the empirical potential would have the same energy as a true valence-electron eigenfunction in the atom He and Kassatochkin applied this concept, in a very crude but surprisingly effective way, to the calculation of cohesive energies and other properties of a large number of non-transition metals. I had occasion to study this work in the course of preparing a review article on thermionic emission in collaboration with m. h. Nichols. and I was very impressed by the possibilities of the method. After some years I called it to the attention of J C. Phillips, who had come to Bell Laboratories in the late 1950s as a postdoctoral worker. When Phillips concluded his stint at Bell Labora. tories, he moved to Berkeley, where in collaboration with a student, Leonard Kleinman, he succeeded in putting the concept of what are now called pseudo potentials on a more logical basis. As is well known, use of pseudopotentials spread like wildfire in the 1960s and their logical basis was still further explored. 2. CORRELATION ENERGY Although people have known from the early days of quantum mechanics that the true ground-state wavefunction of a many-electron system must be of more com plicated form than a determinant of one-electron wavefunctions, and although the importance of the ' correlation-energy'correction to a band-theory calculation of the cohesive energy of a metal had been clearly recognized by Wigner and Seitz 1933 and 1934, the magnitude and role of correlation energy remained inade-