Semiconductors and Covalent Bonding

The metallic bonding described in the previous section is based largely on a delocalization of electrons throughout the entire lattice, yielding, as we have said, 

14) Functionals that include LYP correlation [33] must be viewed with some suspicion 

As we asked before for metals, we now consider what level of accuracy to expect if we choose to perform a self-consistent DFT calculation. The properties considered 

I 2 An Introduction to the Theory of Crystalline Elemental Solids and their Surfaces 

2) The PBE functional predicts lattice constants that are larger than experiment (+0.7 to +2.3%), just as this functional did for metals. The bulk moduli are less than the experimental values (—17 to —5%) and the cohesive energies are within about 4% of experiment, considerably improved compared to LDA. Note that the GGAs for solids tend to yield better lattice constants for these covalent soHds but worse values for [30, 32]. 

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