The negative sign indicates that the amplitude of B1s subtracts from the amplitude
of A1s where they overlap (FIG. 4.26), and there is a nodal surface where the
atomic orbitals cancel completely. In a hydrogen molecule, the nodal surface is a
plane that lies halfway between the two nuclei. If an electron occupies this orbital,
it is largely excluded from the internuclear region and consequently has a higher
energy than when it occupies one of the atomic orbitals alone. A combination of
atomic orbitals that results in an overall raising of energy, like that in Eq. 2, is
called an antibonding orbital.