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Excited-state quantum phase transitions (ESQPTs) are quantum phase transitions that take place at excited levels. They have been shown for different systems with few degrees of freedom. One of such models is the Lipkin-Meshkov-Glick (LMG) model, which has one degree of freedom and is equivalent to a spin-1/2 model with infinite-range interaction. Here, we examine whether the precursors of the ESQPT in the LMG model can still be captured when the long-range interaction in this spin system becomes finite and the Hilbert space exponentially large in system size. [ABSTRACT FROM AUTHOR]

Making use of an algebraic model for the study of diatomic endofullerenes recently published [Physical Review A 94, 032508 (2016)], we present a possible extension of this framework for the calculation of infrared intensities. We apply the model to the absorption infrared spectrum of H2@C60 at T = 6K obtaining satisfactory preliminary results. [ABSTRACT FROM AUTHOR]