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1. Universal anharmonic potential energy surfaces for XY ₂ -type molecules

   https://doi.org/10.1063/5.0096893  

   Sadowsky, Daniel (June 2022, AIP Advances)

   An approach to generate anharmonic potential energy surfaces for both linear and bent XY 2 -type molecules from their equilibrium geometries, Hessians, and total atomization energies alone is presented. Two key features of the potential energy surfaces are that (a) they reproduce the harmonic behavior around the equilibrium geometries exactly and (b) they have the correct limiting behavior with respect to total bond dissociation. The potentials are constructed from two diatomic potentials, for which both the Morse or Varshni potentials are tested, and a triatomic potential, for which modified forms of the Anderson- n potential are tested. Potential energy surfaces for several linear and bent molecules are constructed from ab initio data, and the third-order derivatives of these surfaces at their equilibrium geometries are compared to the results of finite difference computations. For bent molecules, the vibrational spectra predicted by vibrational configuration interaction calculations on these surfaces are compared to experiment. A modified version of the Anderson- n potential, in combination with the Varshni potential, is demonstrated to predict vibrational frequencies associated with bond angle bending an average of 20 cm −1 below the harmonic oscillator approximation and with a fourfold reduction in the root-mean-square deviation from experiment compared to the harmonic oscillator approximation. 

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2. Anharmonic vibrational frequencies of ammonia borane (BH ₃ NH ₃ )

   https://doi.org/10.1063/5.0040050  

   Westbrook, Brent R.; Valencia, E. Michael; Rushing, Spencer C.; Tschumper, Gregory S.; Fortenberry, Ryan C. (January 2021, The Journal of Chemical Physics)

   null (Ed.)
3. Spin–Phonon Interactions and Anharmonic Lattice Dynamics in Fe ₃ GeTe ₂

   https://doi.org/10.1002/apxr.202200089  

   Cai, Qingan; Zhang, Yuemei; Luong, Diana; Tulk, Christopher_A; Fokwa, Boniface_PT; Li, Chen (March 2023, Advanced Physics Research)

   Abstract Raman scattering is performed on Fe₃GeTe₂(FGT) at temperatures from 8 to 300 K and under pressures from the ambient pressure to 9.43 GPa. Temperature‐dependent and pressure‐dependent Raman spectra are reported. The results reveal respective anomalous softening and moderate stiffening of the two Raman active modes as a result of the increase of pressure. The anomalous softening suggests anharmonic phonon dynamics and strong spin–phonon coupling. Pressure‐dependent density functional theory and phonon calculations are conducted and used to study the magnetic properties of FGT and assign the observed Raman modes,and. The calculations proved the strong spin–phonon coupling for themode. In addition, a synergistic interplay of pressure‐induced reduction of spin exchange interactions and spin–orbit coupling effect accounts for the softening of themode as pressure increases. 

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4. Fast Na diffusion and anharmonic phonon dynamics in superionic Na ₃ PS ₄

   https://doi.org/10.1039/D1EE01509E  

   Gupta, Mayanak K.; Ding, Jingxuan; Osti, Naresh C.; Abernathy, Douglas L.; Arnold, William; Wang, Hui; Hood, Zachary; Delaire, Olivier (December 2021, Energy & Environmental Science)

   Strongly anharmonic low-energy phonons enable the fast diffusion of Na ions in the solid-state electrolyte compound Na₃PS₄. 

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5. Anharmonic contribution to the stabilization of Mg(OH) ₂ from first principles

   https://doi.org/10.1039/C8CP02490A  

   Treviño, P.; Garcia-Castro, A. C.; López-Moreno, S.; Bautista-Hernández, A.; Bobocioiu, E.; Reynard, B.; Caracas, R.; Romero, A. H. (January 2018, Physical Chemistry Chemical Physics)

   Geometrical and vibrational characterization of magnesium hydroxide was performed using density functional theory. Four possible crystal symmetries were explored: P 3̄ (No. 147, point group −3), C 2/ m (No. 12, point group 2), P 3 m 1 (No. 156, point group 3 m ) and P 3̄ m 1 (No. 164, point group −3 m ) which are the currently accepted geometries found in the literature. While a lot of work has been performed on Mg(OH) 2 , in particular for the P 3̄ m 1 phase, there is still a debate on the observed ground state crystal structure and the anharmonic effects of the OH vibrations on the stabilization of the crystal structure. In particular, the stable positions of hydrogen are not yet defined precisely, which have implications in the crystal symmetry, the vibrational excitations, and the thermal stability. Previous work has assigned the P 3̄ m 1 polymorph as the low energy phase, but it has also proposed that hydrogens are disordered and they could move from their symmetric position in the P 3̄ m 1 structure towards P 3̄. In this paper, we examine the stability of the proposed phases by using different descriptors. We compare the XRD patterns with reported experimental results, and a fair agreement is found. While harmonic vibrational analysis shows that most phases have imaginary modes at 0 K, anharmonic vibrational analysis indicates that at room temperature only the C 2/ m phase is stabilized, whereas at higher temperatures, other phases become thermally competitive. 

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