Hydrogen modes in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:msub> <mml:mi>KH</mml:mi> <mml:mn>2</mml:mn> </mml:msub> <mml:msub> <mml:mi>PO</mml:mi> <mml:mn>4</mml:mn> </mml:msub> </mml:mrow> </mml:math> under pressure

The nature of the phonon triplet in the region of OH-stretching modes in hydrogen-bonded materials is often explained by the interplay of OH-stretching modes and combinations and overtones of OH-bending modes. To elucidate both contributions in ${\mathrm{KH}}_{2}{\mathrm{PO}}_{4}$ (KDP), we compare the pressure dependence of the OH-bending and stretching modes from ab initio calculations and inelastic neutron scattering (INS) measurements. The ab initio calculations predict a hardening of OH-bending modes and a softening of OH-stretching modes with pressure. At the same time, INS measurements in the region of OH-stretching modes indicate a hardening of the phonon triplet together with the bending modes. This means that this triplet in INS measurements is mainly due to combinations and overtones of OH-bending modes, while the intensity of OH-stretching modes appears to be relatively low. This conclusion may also apply to other hydrogen-bonded materials.