Effects of local and gradient-corrected density approximations on the prediction of the intralayer lattice distance c, in graphite and LiC6

Ab initio total energy calculations, on hexagonal models of graphite and LiC6, are carried out within the most widely density functional theory (DFT) implementation, the local density approximation (LDA). Improvements to LDA in the form of generalized gradient approximation (GGA) are explored. Structural parameters predicted by LDA, as expected underestimate experiment within 1-2% margin of accuracy. GGA does not give a good account in the prediction of lattice parameter c, especially in graphite. This is evident in both recently implemented gradient corrections by Perdew and Wang and earlier corrections by Becke approximations. A substantial improvement is seen on introducing lithium ion in LiC6 and using recent approximations. Valence electron densities from both LDA and GGA calculations, shows charge distribution plots that compare well with experimental results. Charge density distribution plots of these approximations appears similar on a larger scale.