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The potential energy landscape (PEL) formalism is a tool within statistical mechanics that has been used in the past to calculate the equation of states (EOS) of classical rigid model liquids at low temperatures, where computer simulations may be challenging. In this work, we use classical molecular dynamics (MD) simulations and the PEL formalism to calculate the EOS of the flexible q-TIP4P/F water model. This model exhibits a liquid–liquid critical point (LLCP) in the supercooled regime, at (Pc = 150 MPa, Tc = 190 K, and ρc = 1.04 g/cm3) [using the reaction field technique]. The PEL-EOS of q-TIP4P/F water and the corresponding location of the LLCP are in very good agreement with the MD simulations. We show that the PEL of q-TIP4P/F water is Gaussian, which allows us to calculate the configurational entropy of the system, Sconf. The Sconf of q-TIP4P/F water is surprisingly similar to that reported previously for rigid water models, suggesting that intramolecular flexibility does not necessarily add roughness to the PEL. We also show that the Adam–Gibbs relation, which relates the diffusion coefficient D with Sconf, holds for the flexible q-TIP4P/F water model. Overall, our results indicate that the PEL formalism can be used to study molecular systems that include molecular flexibility, the common case in standard force fields. This is not trivial since the introduction of large bending/stretching mode frequencies is problematic in classical statistical mechanics. For example, as shown previously, we find that such high frequencies lead to unphysical (negative) entropy for q-TIP4P/F water when using classical statistical mechanics (yet, the PEL formalism can be applied successfully). [ABSTRACT FROM AUTHOR]

Technetium-99, a prevalent by-product of the nuclear fuel cycle, is a transition metal with 9 accessible oxidation states and a half-life of 2.1 × 105 years. 99TcVII has been found in the environment surrounding National Lab sites and fuel reprocessing centers, and its separation and long-term storage is complicated by its extensive redox chemistry. In our prior work we have reported the synthesis and reduction of TcVO to TcIVO in α1- and α2-Wells–Dawson P2W17O6110– polyoxometalates and reported on the polyoxometalate features that promote reduction and stabilization. In this work we report on the oxidation state stabilization of TcVO, TcVIO and TcVIN within the α-Keggin PW11O397– polyoxometalate. Both Density Functional Theory and experimental results show that Tc in TBA4[TcVIN-PW11O39] can be reduced to TcV, but any subsequent electrons reduce the tungsten framework of the polyoxometalate. TBA4[TcVO-PW11O39] however, can be theoretically reduced down to TcI before reduction of the polyoxometalate occurs.

Tuning the properties of magnetic topological materials is of interest to realize exotic physical phenomena, new quantum phases and quasiparticles, and topological spintronic devices. However, current topological materials exhibit Curie temperature (TC) values far below those needed for practical applications. In recent years, significant progress has been made to control and optimize TC, particularly through defect-engineering of these structures. Most recently, we reported TC values up to 80 K for (MnSb2Te4)x(Sb2Te3)1−x when 0.7 ≤ x ≤ 0.85 by controlling the composition x and the Mn content in these structures during molecular beam epitaxy growth. In this study, we show further enhancement of the TC, as high as 100 K, by maintaining high Mn content and reducing the growth rate from 0.9 nm/min to 0.5 nm/min. Derivative curves of the Hall resistance and the magnetization reveal the presence of two TC components contributing to the overall value and suggest TC1 and TC2 have distinct origins: excess Mn in MnSb2Te4 septuple layers (SLs) and high Mn content in Sb2−yMnyTe3 quintuple layer (QL) alloys, respectively. To elucidate the mechanisms promoting higher TC values in this system, we show evidence of enhanced structural disorder due to the excess Mn that occupies not only Sb sites but also Te sites, leading to the formation of a new crystal structure for these materials. Learning to control defects that enhance desired magnetic properties and understanding the mechanisms that promote high TC in magnetic topological materials such as (Mn1+ySb2−yTe4)x(Sb2−yMnyTe3)1−x is of great importance to achieve practical quantum devices. [ABSTRACT FROM AUTHOR]

As a liquid approaches the gas state, the properties of the potential energy landscape (PEL) sampled by the system become anomalous. Specifically, (i) the mechanically stable local minima of the PEL [inherent structures (IS)] can exhibit cavitation above the so-called Sastry volume, vS, before the liquid enters the gas phase. In addition, (ii) the pressure of the liquid at the sampled IS [i.e., the PEL equation of state, PIS(v)] develops a spinodal-like minimum at vS. We perform molecular dynamics simulations of a monatomic water-like liquid and verify that points (i) and (ii) hold at high temperatures. However, at low temperatures, cavitation in the liquid and the corresponding IS occurs simultaneously and a Sastry volume cannot be defined. Remarkably, at intermediate/high temperatures, the IS of the liquid can exhibit crystallization, i.e., the liquid regularly visits the regions of the PEL that belong to the crystal state. The model liquid considered also exhibits a liquid–liquid phase transition (LLPT) between a low-density and a high-density liquid (LDL and HDL). By studying the behavior of PIS(v) during the LLPT, we identify a Sastry volume for both LDL and HDL. The HDL Sastry volume marks the onset above which IS are heterogeneous (composed of LDL and HDL particles), analogous to points (i) and (ii) above. However, the relationship between the LDL Sastry volume and the onset of heterogeneous IS is less evident. We conclude by presenting a thermodynamic argument that can explain the behavior of the PEL equation of state PIS(v) across both the liquid–gas phase transition and LLPT. [ABSTRACT FROM AUTHOR]

We perform path integral molecular dynamics (PIMD) simulations of a monatomic liquid that exhibits a liquid–liquid phase transition and liquid–liquid critical point. PIMD simulations are performed using different values of Planck's constant h, allowing us to study the behavior of the liquid as nuclear quantum effects (NQE, i.e., atoms delocalization) are introduced, from the classical liquid (h = 0) to increasingly quantum liquids (h > 0). By combining the PIMD simulations with the ring-polymer molecular dynamics method, we also explore the dynamics of the classical and quantum liquids. We find that (i) the glass transition temperature of the low-density liquid (LDL) is anomalous, i.e., T g LDL (P) decreases upon compression. Instead, (ii) the glass transition temperature of the high-density liquid (HDL) is normal, i.e., T g HDL (P) increases upon compression. (iii) NQE shift both T g LDL (P) and T g HDL (P) toward lower temperatures, but NQE are more pronounced on HDL. We also study the glass behavior of the ring-polymer systems associated with the quantum liquids studied (via the path-integral formulation of statistical mechanics). There are two glass states in all the systems studied, low-density amorphous ice (LDA) and high-density amorphous ice (HDA), which are the glass counterparts of LDL and HDL. In all cases, the pressure-induced LDA–HDA transformation is sharp, reminiscent of a first-order phase transition. In the low-quantum regime, the LDA–HDA transformation is reversible, with identical LDA forms before compression and after decompression. However, in the high-quantum regime, the atoms become more delocalized in the final LDA than in the initial LDA, raising questions on the reversibility of the LDA–HDA transformation. [ABSTRACT FROM AUTHOR]

Technetium‐99, a prevalent by‐product of the nuclear fuel cycle, is a transition metal with 9 accessible oxidation states and a half‐life of 2.1 × 105 years. 99TcVII has been found in the environment surrounding National Lab sites and fuel reprocessing centers, and its separation and long‐term storage is complicated by its extensive redox chemistry. In our prior work we have reported the synthesis and reduction of TcVO to TcIVO in α1‐ and α2‐Wells–Dawson P2W17O6110– polyoxometalates and reported on the polyoxometalate features that promote reduction and stabilization. In this work we report on the oxidation state stabilization of TcVO, TcVIO and TcVIN within the α‐Keggin PW11O397– polyoxometalate. Both Density Functional Theory and experimental results show that Tc in TBA4[TcVIN‐PW11O39] can be reduced to TcV, but any subsequent electrons reduce the tungsten framework of the polyoxometalate. TBA4[TcVO‐PW11O39] however, can be theoretically reduced down to TcI before reduction of the polyoxometalate occurs. [ABSTRACT FROM AUTHOR]

Heat capacities are calculated as a function of temperature for bimetallic clusters composed of six palladium and seven nickel atoms using Monte Carlo techniques both with and without J-walking. By applying a simulated annealing strategy, the minimum energy configurations at 0 K are identified for a series of interatomic interaction strengths. A significant dependence of the spatial arrangement of atoms on the strength of the Pd–Ni interaction is observed. Calculations of the heat capacity as a function of temperature show the presence of a ‘‘melting’’ coexistence region similar to that observed in pure clusters. For the alloy clusters, low temperature heat capacity anomalies are observed arising from isomerizations that are reminiscent of order–disorder transitions known to occur in some bulk alloy materials. These low temperature heat capacity anomalies are observable only when the J-walking algorithm is used. [ABSTRACT FROM AUTHOR]

Changes in Gibbs free energies for the process Arn-1,(a) + Ar(a) → Arn,(a) are calculated by Monte Carlo simulations for Ar clusters physisorbed on a graphite substrate. Calculations are performed for clusters Ar2 through Ar12 at 10 K. Using a simulated annealing procedure, the minimum-energy configurations at 0 K are obtained. In all cases studied minimal-energy two-dimensional structures are found at a distance above the surface identical to that determined for an argon monomer. Some cluster sizes exhibit isomerization between several low-energy configurations during the simulations. This isomerization leads to sampling difficulties that are significantly reduced by using the J-walking method. Minima in the Gibbs free energy are found for cluster sizes 7, 10, and 12. An explanation for the location of the minima in the free-energy function is given in terms of cluster structure and energetics. [ABSTRACT FROM AUTHOR]

Investigates the phase transitions in rare-gas clusters adsorbed on model surfaces using Monte Carlo technique. Calculation of heat capacities as a function of temperature; Generation of the minimum-energy configuration at zero kelvin; Influence of high-energy isomers on the corrugated surface.

Investigates the quantum dissociation dynamics of hydrogen on a rigid nickel cluster using a quantum mechanical model. Basis of the model on the spectral grid/fast Fourier transform technique; Inclusion of three variables which are treated quantum mechanically; Factors influencing the remaining three variables fixed during simulations.

Studies the phase transitions in molecular cluster using classical Monte Carlo techniques. Usual solid-liquid coexistence region; Low-temperature phase transition associated with an orientational change in the molecular components of the system; Use of root-mean-square bond fluctuations and distribution of potential energies to characterize various phase transitions.

Using the density functional theory, the electronic structure of [image omitted] (R=H, CH2CH3) complexes was determined and the binding energy calculated. For [image omitted] and [image omitted] the ligands are bound to the iron in bidentate coordination, whereas [image omitted] forms an iron oxide bond that might be the precursor in iron oxide nanoparticle synthesis. Large binding energies were found, with particular stability obtained for [image omitted] due to the formation of an iron oxide bond. The iron oxide bonds were clearly identified by computing the charge density isosurfaces of the relevant molecular orbitals. Using the molecular partition functions obtained from the ab-initio results, the stability of each system was determined as a function of temperature. In all cases, the systems are thermodynamically stable up to very high temperatures. [ABSTRACT FROM AUTHOR]

By implementing the J-walking formalism within the test particle method, adsorption isotherms for various Lennard-Jones systems were constructed. The method decreases the uncertainties in the average thermodynamic values by approximately one order of magnitude and substantially increases the percentage insertion of the ghost particle. The adsorption isotherms showed that at low temperatures the amount of adsorbed material seems to be independent of the vapour pressure of the gas if attractive forces dominate the system. On the other hand, systems dominated by repulsive forces show the usual type B behaviour for adsorption isotherms. The equation of state is used to confirm these results. [ABSTRACT FROM AUTHOR]