Single-ion conducting solid electrolytes are gaining tremendous attention as essential materials for solidstate batteries, but a comprehensive understanding of the factors that dictate high ion mobility remains elusive. Here, for the first time, we use a combination of the Maximum Entropy Method analysis of room-temperature neutron powder diffraction data, ab initio molecular dynamics, and joint-time correlation analysis to demonstrate that the dynamic response of the anion framework plays a significant role in the new class of fast ion conductors, Na 11 Sn 2 PnX 12 (Pn = P, Sb; X = S, Se). Facile [PX 4 ] 3− anion rotation exists in superionic Na 11 Sn 2 PS 12 and Na 11 Sn 2 PSe 12 , but greatly hindered [SbS 4 ] 3− rotational dynamics are observed in their less conductive analogue, Na 11 Sn 2 SbS 12 . Along with introducing dynamic frustration in the energy landscape, the fluctuation caused by [PX 4 ] 3− anion rotation is firmly proved to couple to and facilitate long-range cation mobility, by transiently widening the bottlenecks for Na + -ion diffusion. The combined analysis described here resolves the role of the long-debated paddle-wheel mechanism, and is the first direct evidence that anion rotation significantly enhances cation migration in rotor phases. The joint-time correlation analysis developed in our work can be broadly applied to analyze coupled cation−anion interplay where traditional transition state theory does not apply. These findings deliver important insights into the fundamentals of ion transport in solid electrolytes. Invoking anion rotational dynamics provides a vital strategy to enhance cation conductivity and serves as an additional and universal design principle for fast ion conductors.

A combination of the maximum entropy method and AIMD simulations demonstrates that polyanion [PS 4 ] 3À rotation is facile in the fast ion conductors b-Li 3 PS 4 and its Si-substituted analog, Li 3.25 Si 0.25 P 0.75 S 4 , but absent in the nonconductive phase, g-Li 3 PS 4 . The increased entropy upon the substitution of Si for P stabilizes the high-temperature rotor phase (b-Li 3 PS 4 ) at room temperature. Jointtime correlation analysis and AIMD simulations show that [PS 4 ]/[SiS 4 ] anion rotational dynamics are coupled to and greatly enhance cation diffusion by widening the bottleneck for Li + -ion transport.

We use the path integral ground state method to study the energetic and structural properties of small para-H2 clusters of sizes ranging from 2 to 20 molecules. A fourth order formula is used to approximate the short imaginary-time propagator and two interaction potentials are considered. Our results are compared to those of exact basis set calculations and other quantum Monte Carlo methods when available. We find that for all cluster sizes considered, our results show a lower ground state energy than literature values obtained by diffusion Monte Carlo and variational Monte Carlo. For the dimer and trimer, ground state energies are in good agreement with exact results obtained using the discrete variable representation. Structural properties are found to be insensitive to the choice of interaction potential. We explore the use of Pekeris coordinates to analyze the importance of linear arrangement in trimers and for trimers within clusters of larger size.

Combining semiclassical time evolution and quantum Boltzmann operator to evaluate reactive flux correlation function for thermal rate constants of complex systems A relationship between centroid dynamics and path integral quantum transition state theoryThe centroid dynamics formalism is extended to the calculation of time correlation functions of nonlinear operators. It is shown that centroid correlation functions can be related to quantum mechanical ones via higher order Kubo-type transforms. A key step is the construction of the correlation functions from a mixed classical/semiclassical centroid representation of the operators. A general methodology is developed to relate these Kubo-type transforms to the desired quantum correlation functions. The approach is tested using a one-dimensional anharmonic potential for which the ͗x 2 x 2 (t)͘ and the ͗x 3 x 3 (t)͘ correlation functions are computed. Applications of this new approach are also outlined.

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