The DFT methods had the largest MAX error from B 3 H 3 and B 2 H 2 . BEBOP energy for HBNH, however, deviates the most since ionic and charge transfer are critical in this structure. The BEBOP result for Be 4 is encouraging, as it suggests that bonding in metal clusters might be amenable to a BEBOP analysis. Nevertheless, the BEBOP description of the three-center two-electron bonds of diborane appears to be qualitatively correct and is not significantly altered by rescaling (by 1.052) to fit the ROCBS-QB3 energy (Figure ).