Excited state quantum phase transitions in the bending spectra of molecules.

• Modeling of the experimental bending rovibrational spectra data of a set of molecular species which encompasses the full gamut of structures: semirigid linear (HNC), semirigid bent (H2S) and nonrigid (Si2C and NCNCS). • Most general Hamiltonian of the two dimensional limit of the vibron model including all possible interactions up to four-body. • Predictions of the higher excited rovibrational energies are provided. • Analysis of the molecular dynamical structure using the quantum monodromy diagram, the Birge-Sponer plot and the participation ratio. We present an extension of the Hamiltonian of the two dimensional limit of the vibron model to encompass all possible interactions up to four-body operators. We apply this Hamiltonian to the modeling of the bending spectrum of four molecules: HNC, H 2 S, Si 2 C, and NCNCS. The selected molecular species include linear, bent, and nonrigid equilibrium structures, proving the versatility of the algebraic approach which allows for the consideration of utterly different physical cases within a single Hamiltonian and a general formalism. For each case we compute predicted bending energies and wave functions, that we use to depict the associated quantum monodromy diagram, Birge-Sponer plot, and participation ratio. In nonrigid cases, we also show the bending energy functional obtained using the coherent –or intrinsic– state formalism. [ABSTRACT FROM AUTHOR]