A semi-empirical method that eliminates three- and four-center two-electron integrals is named for the “neglect” of one form of this phenomenon. For the H2+ (“H-2-plus”) ion, a pair of energies are given by “H-AA plus H-AB over quantity one plus” a measure of this phenomenon. Roothaan (“ROTE-hahn”) simplified the Hartree–Fock method into the equation “FC equals SC epsilon,” where S is a matrix named for this phenomenon. For two wavefunctions, this phenomenon is described by the integral over all volume of “psi-A-star times psi-B,” which vanishes for orthogonal orbitals like p-x and p-y. A pair of atomic orbitals for which this phenomenon is higher will have more stabilized bonding molecular orbitals. Sigma bonds form when this phenomenon is end-on. (10[1])For 10 points, name this phenomenon in which space is shared by two orbitals. ■END■ (0[4])