The role of the alkali and chalcogen atoms on the stability of the layered chalcogenide A2M(II)M(IV)3Q3 (A=alkali; M+ metal-cations; Q=clacogen) compounds: a density functional theory investigation within van der Waals corrections - Besse, Rafael; Silva, Juarez Lopes Ferreira da (Journal of Physics: Condensed Matter, 2017)