Fig. 2.
Example of exchange-potential nonadditivity using the LDA exchange functional in molecular hydrogen. The hydrogen atom density is the quantum mechanically exact density, and the atoms are separated by 1.4011 Bohr. The x-axis is the position along the internuclear axis. The sum of the isolated atom exchange-potentials (dashed green line) is more negative than the molecular exchange-potential (solid black line). At the bond midpoint (r=0), the ratio of the summed potentials to the molecular potential is exactly 2 : 21/3