Signatures of quantum phase transitions and excited state quantum phase transitions in the vibrational bending dynamics of triatomic molecules

Signatures of quantum phase transitions (QPTs) and excited state quantum phase transitions (ESQPTs) in the bending motion of eight XYZ triatomic molecules (HCN, HNC, NiCN; CaOH, CaOD, MgOH, MgOD and OCS), the large-amplitude bending degree of freedom of a tetratomic molecule (HNCS), and four symmetric XY2 triatomic molecules (H2O, D2O, H2S, H2Se) are investigated in an attempt to understand their dependence on the composition of the molecular species. It is shown that the isomerizing HCN/HNC system leads to quasi-linearity, that the water molecule H2O displays a dramatic effect at Ex ∼ 11,000 cm−1 with clear indication of an ESQPT at about vibrational quantum number υb = 8, and that the heavy water molecule D2O is expected to undergo an ESQPT at υb = 10.