The atom-atom interaction model has been extended to the case of $2H-\mathrm{Mo}{\mathrm{S}}_2$ and it is shown that the observed vibrational frequencies can be satisfactorily reproduced by only three force constants. However, the calculation leads to a frequency reversal in the Davydov pair which is very similar to that found earlier for $\epsilon -\mathrm{GaSe}$ and $\beta -\mathrm{GaS}$. Since Mo${\mathrm{S}}_2$ is somewhat less ionic than the two gallium compounds, we suggest that the frequency reversal may not be due to the long-range interactions proposed earlier. We discuss the possible role of short-range multipolar interactions.

• Received 4 November 1982

DOI:https://doi.org/10.1103/PhysRevB.28.2237