Cao and coworkers, 2018, "in some Raman experiments", "observed Raman-forbidden modes above the position of the A1g mode, which [the researchers] tentatively assigned to out of plane A2u and B2g infrared and inactive modes", according to Molina-Sánchez and Wirtz, 2011. Molina-Sánchez and Wirtz proposed ab-initio calculations of the phonon dispersions in the case of single-layer and bulk MoS2 and WS2, exploring the Raman active modes [Molina-Sánchez and Wirtz] explain this decrease by an enhancement of the dielectric screening of the long-range Coulomb interaction between the effective charges with growing number of layers. This decrease in the long-range part over-compensates the increase of the short-range interaction due to the weak inter-layer interaction" (Molina-Sánchez and Wirtz, 2011).