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 Li C6, 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. Struc tural parameters predicted by LDA, as expected underestimate experiment wit hin 1-2% margin of accuracy. GGA does not give a good account in the predic tion of lattice parameter c, especially in graphite. This is evident in bot h recently implemented gradient corrections by Perdew and Wang and earlier corrections by Becke approximations. A substantial improvement is seen on i ntroducing lithium ion in LiC6 and using recent approximations. Valence ele ctron densities from both LDA and GGA calculations, shows charge distributi on plots that compare well with experimental results. Charge density distri bution plots of these approximations appears similar on a larger scale.