supplement information (SI)

The supplementary material contains the results of band structure of Mg2C and Mg2Si, and the optical spectra (ε_2,ε_1,n,k and R) of Mg2C without considering local field effects. The tabulated comparison of energy peak positions of ε_2, k and R with LFE calculated using RPA, LRC and BS kernels are presented. The peak positions of EEL spectra computed using RPA and three XC kernels, with and without incorporating LFE are also included in the supplementary material.