ABSTRACT
Ikerbasque Research Professor, Donostia International Physics Center (DIPC), 20018 Donostia, Euskadi, Spain; Euskal Herriko Unibertsitatea (UPV/EHU), 20018 Donostia, Euskadi, Spain; Basque Foundation for Science (IKERBASQUE), 48011 Bilbao, Euskadi, Spain, E-mail: mario.piris@ehu.eus
Even the formal justi¿cation of the electron-pair bond in the simplest cases ... requires a formidable array of symbols and equations.
