Effect of the basis set selection on the electron momentum distribution in graphite

This work evaluates three spatial distribution functions of the Bernal graphite momentum: the linear electron momentum distribution LEMD, the Compton profile function CP, and the self-correlation function or reciprocal form function B(r). We used the CRYSTAL software with a restricted Hartree-Fock Hamiltonian to obtain an ab initio and self-consistent crystalline wavefunction, whose square module, integrated over the first Brillouin zone and added over the occupied orbitals, defines the electron momentum density. The effect of different basis sets on these functions is reviewed. We have used modified STO-3GM, 3-21GM, and 6-31GM basis sets and standard basis sets with the same designation. Our results reveal the preponderance of pz and s orbitals in the electron momentum in the 〈001〉 direction, and px and s orbitals in the 〈100〉 direction. The Compton profile for the core and valence electrons presents significant differences in the limit of small vectors. The results obtained by using the standard basis sets differ substantially from those obtained in this work. They do so, basically because of the failure of the former to reproduce the adequate Fermi level, something that is underscored via anisotropy curves of the linear electron momentum density and the Compton profile in the directions studied. Our calculations overestimate the Compton profile anisotropy between directions 〈001〉 and 〈100〉.