ABSTRACT
PRELIMINARY REMARKS: Sixty years ago, Rudy Pariser and Bob Parr founded a new π-electron theory. It was ultimately called the Pariser-Parr-Pople (PPP) theory, in which there were parameters, including the difference between the ionization potential and electron afnity, I – A. Much followed.1 When the editors of the Deb book invited Parr to contribute, he thought I – A would make an interesting subject, and he secured the coauthorship of Pariser. He also shared his idea with one of the editors of this volume, Swapan Ghosh. To our surprise, Ghosh very soon produced a lucid and elegant rst draft. This draft, in its entirety, with only minor changes by us, is reproduced as the full text below. This is an editor’s major gift of authorship, of course, and we thank Ghosh profusely.
