Molecular-bond-energy calculations based on the Harris-functional approximation coupled with the generalized-gradient approximation

We have developed a molecular-orbital (MO) calculational method, based on the Harris-functional approximation coupled with the generalized-gradient approximation (Harris-GGA), in order to get reasonable molecular-bond energies of large systems for which the Kohn-Sham self-consistent-field (SCF) calculation is impractical. It has been applied to some diatomic molecules and three types of cage-shaped carbon cluster. For the diatomic molecules ${\mathrm{Li}}_2$, ${\mathrm{Be}}_2$, ${\mathrm{C}}_2$, ${\mathrm{N}}_2$, ${\mathrm{F}}_2$, ${\mathrm{Cu}}_2$, and CO, the bond-energy differences between the Harris-GGA and experimental values are 30–50 % less than those between the Harris-functional approximation with a local-density approximation (Harris-LDA) and experimental values. For the carbon clusters, ${\mathrm{C}}_8$ and ${\mathrm{C}}_{24}$, the bond energies calculated with use of the Harris-GGA are in agreement with those calculated with use of the SCF-GGA within about 10%. For the ${\mathrm{C}}_{60}$ fullerene, the calculated bond lengths and highest-occupied-MO–lowest-unoccupied-MO gap energy are comparable to experimental values.