Accurate calculation of zero point energy from molecular dynamics simulations of liquids and their mixtures

Exploration of Diverse Interactions of l-Methionine in Aqueous Ionic Liquid Solutions: Insights from Experimental and Theoretical Studies

Here, we have investigated some physicochemical parameters to understand the molecular interactions by means of density (ρ) measurement, measurement of viscosity (η), refractive index(n _D) measurement, and conductance and surface tension measurements between two significant aqueous ionic liquid solutions: benzyl trimethyl ammonium chloride (BTMAC) and benzyl triethyl ammonium chloride (BTEAC) in an aqueous l-methionine (amino acid) solution. The apparent molar volume (Φ_v), coefficient of viscosity (B), and molar refraction (R _M) have been used to analyze the molecular interaction behavior associated in the solution at various concentrations and various temperatures. With the help of some important equations such as the Masson equation, the Jones-Doles equation, and the Lorentz-Lorenz equation, very significant parameters, namely, limiting apparent molar volumes (Φ_v ⁰ ), coefficient of viscosity (B), and limiting molar refraction (R _M ⁰), respectively, are obtained. These parameters along with specific conductance (κ) and surface tension (σ) are very much helpful to reveal the solute-solvent interactions by varying the concentration of solute molecules and temperature in the solution. Analyses of Δμ₁ ^0#, Δμ₂ ^0#, TΔS ₂ ^0#, ΔH ₂ ^0#, and thermodynamic data provide us valuable information about the interactions. We note that l-Met in 0.005 molality BTEAC ionic liquid at 308.15 K shows maximum solute-solvent interaction, while l-Met in 0.001 molality BTMAC aqueous solution of ionic liquid at 298.15 K shows the minimum one. Spectroscopic techniques such as Fourier transform infrared (FTIR), ¹H-NMR, and UV-vis also provide supportive information about the interactions between the ionic liquid and l-methionine in aqueous medium. Furthermore, adsorption energy, reduced density gradient (RDG), and molecular electrostatic potential (MESP) maps obtained by the application of density functional theory (DFT) have been used to determine the type of interactions, which are concordant with the experimental observations.

DFT and TD-DFT study of hydrogen bonded complexes of aspartic acid and n water (n = 1 and 2)

CONTEXT

Hydrogen bonds (HB) influence the conformational preferences of biomolecules and their optical and electronic properties. The directional interaction of molecules of water can be a prototype to understand the effects of HBs on biomolecules. Among the neurotransmitters (NT), L-aspartic acid (ASP) stands out due to its importance in health and as a precursor of several biomolecules. As it presents different functional groups and readily forms inter- and intramolecular HBs, ASP can be considered a prototype for understanding the behavior of NTs when interacting by HB with other substances. Although several theoretical studies have been performed in the past on isolated ASP and its formed complexes with water, both in gas and liquid phases, using DFT and TD-DFT formalisms, these works did not perform large basis set calculations or study electronic transitions of ASP-water complexes. We investigated the HB interactions in complexes of ASP and water molecules. The results show that the interactions between the carboxylic groups of ASP with water molecules, forming cyclic structures with two HBs, lead to more stable and less polar complexes than other conformers formed between water and the NH₂ group. It was observed that there is a relationship between the deviation in the UV-Vis absorption band of the ASP and the interactions of water with the HOMO and LUMO orbitals with the stabilization/destabilization of the S₁ state to the S₀ of the complexes. However, in some cases, such as 1:1 complex ASP-W2, this analysis may be inaccurate due to small changes in ΔE.

METHODS

We studied the landscapes of the ground state surface of different conformers of isolated L-ASP and the L-ASP-(H₂O)_n complexes (n = 1 and 2) using the DFT formalism, with the B3LYP functional, and six different basis sets: 6-31 +  + G(d,p), 6-311 +  + G(d,p), D95 +  + (d,p), D95V +  + (d,p), cc-pVDZ, and, cc-pVTZ basis sets. The cc-pVTZ basis set provides the minimum energy of all conformers, and therefore, we performed the analysis with this basis set. We evaluated the stabilization of the ASP and complexes using the minimum ground state energy, corrected by the zero point energy and the interaction energy between the ASP and the water molecules. We also calculated the vertical electronic transitions S₁ ← S₀, and their properties using the TD-DFT formalism at B3LYP/cc-pVTZ level with the optimized geometries for S₀ state with the same basis set. For the analysis of the vertical transitions of isolated ASP and the ASP-(H₂O)_n complexes, we calculated the electrostatic energy in the S₀ and S₁ states. We performed the calculations with the Gaussian 09 software package. We used the VMD software package to visualize the geometries and shapes of the molecule and complexes.

Flexible machine-learning interatomic potential for simulating structural disordering behavior of Li7La3Zr2O12 solid electrolytes

Batteries based on solid-state electrolytes, including Li₇La₃Zr₂O₁₂ (LLZO), promise improved safety and increased energy density; however, atomic disorder at grain boundaries and phase boundaries can severely deteriorate their performance. Machine-learning (ML) interatomic potentials offer a uniquely compelling solution for simulating chemical processes, rare events, and phase transitions associated with these complex interfaces by mixing high scalability with quantum-level accuracy, provided that they can be trained to properly address atomic disorder. To this end, we report the construction and validation of an ML potential that is specifically designed to simulate crystalline, disordered, and amorphous LLZO systems across a wide range of conditions. The ML model is based on a neural network algorithm and is trained using ab initio data. Performance tests prove that the developed ML potential can predict accurate structural and vibrational characteristics, elastic properties, and Li diffusivity of LLZO comparable to ab initio simulations. As a demonstration of its applicability to larger systems, we show that the potential can correctly capture grain boundary effects on diffusivity, as well as the thermal transition behavior of LLZO. These examples show that the ML potential enables simulations of transitions between well-defined and disordered structures with quantum-level accuracy at speeds thousands of times faster than ab initio methods.

Beyond Thermodynamics: Assessing the Dynamical Softness of Hydrated Ions from First Principles

Ion (de)hydration is a key rate-determining step in interfacial processes from corrosion to electrochemical energy storage. However, predicting the kinetics of ion (de)hydration remains challenging, prompting the use of static proxies such as hydration energy and valence. While useful for assessing thermodynamic preferences, such descriptors cannot fully capture the dynamical softness of the hydration shell that dictates kinetics. Accordingly, we use first-principles molecular dynamics to analyze hydration shell softness for a diverse set of metal cations. Three dynamic metrics are introduced to intuitively describe the bond rigidity, shape deformability, and exchange fluidity of the solvation shell. Together, these metrics capture the relevant physics in the static descriptors, while offering a far more complete and efficient representation for the overall propensity for (de)hydration. Application to the hydrated ion set demonstrates a weak connection between dynamical softness and hydration energy, confirming that dynamical descriptors of hydration are key for correctly describing ion transfer processes.

Nuclear Quantum Effects from the Analysis of Smoothed Trajectories: Pilot Study for Water

Nuclear quantum effects have significant contributions to thermodynamic quantities and structural properties; furthermore, very expensive methods are necessary for their accurate computation. In most calculations, these effects, for instance, zero-point energies, are simply neglected or only taken into account within the quantum harmonic oscillator approximation. Herein, we present a new method, Generalized Smoothed Trajectory Analysis, to determine nuclear quantum effects from molecular dynamics simulations. The broad applicability is demonstrated with the examples of a harmonic oscillator and different states of water. Ab initio molecular dynamics simulations have been performed for ideal gas up to the temperature of 5000 K. Classical molecular dynamics have been carried out for hexagonal ice, liquid water, and vapor at atmospheric pressure. With respect to the experimental heat capacity, our method outperforms previous calculations in the literature in a wide temperature range at lower computational cost than other alternatives. Dynamic and structural nuclear quantum effects of water are also discussed.

Time dependence of quantum correlation functions

In the past few years, the exponential expansion analysis of time autocorrelation functions has provided profound insight into the leading microscopic processes driving the atomic-scale dynamics and has made it possible to highlight the presence and the role of various relaxation channels through which the fundamental correlation functions decay with time. Here we apply this method to the determination of the full time dependence of a correlation function c(t) in a quantum system at nonzero temperature, by making explicit its relationship with its Kubo transform c_{K}(t), which in some cases can be approximately computed with the presently available quantum simulation techniques. We obtain an exact expression for c(t) in terms of the exponential modes that describe the time behavior of c_{K}(t). The relative importance of the various modes in determining the overall shape of c(t) can then be studied in detail. This work extends to the full time domain the results of a previous paper [Guarini et al., Phys. Rev. Lett. 123, 135301 (2019)PRLTAO0031-900710.1103/PhysRevLett.123.135301], in which we employed the same method to calculate the zero time value of the velocity autocorrelation function, to obtain a microscopic description of the quantum mean kinetic energy in a fluid. In particular, we show that the decay constants and the frequencies of the dominant microscopic modes of c(t) are the same as those of c_{K}(t), but the dynamics of the quantum system also contains an additional term decaying on a time scale determined solely by temperature of the system.