A combination of high-sensitivity Raman scattering and ab initio computer simulations is used to explore the lattice dynamics of the prototypical layered semiconductor GeS under hydrostatic pressure. The observed and calculated pressure responses of the Ag layer shear mode are in excellent agreement over the entire pressure range of the experiments (0 to 50 kbar). Examination of the calculated phonon eigenvectors reveals that the "rigid-layer" model is an appropriate description of the lattice dynamics only under near-ambient-pressure conditions and that substantial mode admixture occurs under compression.
