Effects Of Vibrational Anharmonicity Research Articles

The thermoelastic properties of monolayer covalent organic frameworks studied by machine-learning molecular dynamics.

Two-dimensional (2D) covalent organic frameworks (COFs) are emerging as promising 2D polymeric materials with broad applications owing to their unique properties, among which the mechanical properties are quite important for various applications. However, the mechanical properties of 2D COFs have not been systematically studied yet. Herein, a machine-learned neuroevolution potential (NEP) was developed to study the elastic properties of two representative monolayer 2D COFs, namely COF-1 and COF-5. The trained NEP enables one to study the elastic properties of 2D COFs in realistic situations (e.g., finite size and temperature) and possesses greatly improved computational efficiency when compared with density functional theory calculations. With the aid of the obtained NEP, molecular dynamics (MD) simulations together with a strain-fluctuation method were employed to evaluate the elastic constants of the considered 2D COFs at different temperatures. The elastic constants of COF-1 and COF-5 monolayers were found to decrease with an increase in the temperature, though they were almost isotropic irrespective of the temperature. The thermally induced softening of 2D COFs below a critical temperature was observed, which is mainly attributed to their inherent ripple configurations at finite temperatures, while above the critical temperature, the damping effect of anharmonic vibrations became the dominant factor. Based on the proposed mechanisms, analytical models were developed for capturing the temperature dependence of elastic constants, which were found to agree with the MD simulation results well. This work provides an in-depth insight into the thermoelastic properties of monolayer COFs, which can guide the development of 2D COF materials with tailored mechanical behaviors for enhancing their performance in various applications.

Temperature dependence of generalized stacking fault free energy profiles and dissociation mechanisms of slip systems in Mg

A machine learning-derived interatomic potential coupled with the Projected Average Force Integrator (PAFI) method was utilized to examine the temperature-dependent Generalized Stacking Fault Free Energies (GSFFE) profiles across all slip systems in Mg, fully accounting for anharmonic thermal vibrations. Intrinsic Stacking Fault Free Energies (ISFFE) and Unstable Stacking Fault Free Energies (USFFE) were assessed for temperatures ranging from 0 K to 800 K, marginally below the anticipated Mg melting temperature of approximately 900 K. The findings revealed that anharmonic vibrational effects are comparable to harmonic effects for Basal slip above 200 K, approximately half the Debye temperature. Anharmonic contributions were minimal for non-basal slip systems. The USFFEs of Pyramidal-I and II slip systems are predicted to crossover around 385 K due to vibrational effects. Marginally higher ISFFEs in Pyramidal-I across all temperatures also indicate possible cross-slip or SF transformation between the systems. A comprehensive examination of slip trajectories and atomic displacements unveiled intricate atomic displacements across all Mg slip systems and complex shuffle mechanisms for non-basal slips. In addition to corroborating previously documented dislocation dissociation mechanisms for Basal, Pyramidal-〈a〉, and Pyramidal-I slips, a new dissociation mechanism was discerned for dislocation slip in Pyramidal-II. This study offers crucial insights into the temperature-dependent behavior of SFFEs in Mg, the significance of intrinsic anharmonic thermal contributions to Basal slip, and the intricate atomic displacements and dissociation mechanisms of hexagonal close packed slip systems.

Exploring anharmonic lattice dynamics and dielectric relations in niobate perovskites from first-principles self-consistent phonon calculations

Group I niobates (KNbO3 and NaNbO3) are promising lead-free alternatives for high-performance energy storage applications. Despite their potential, their complex phase transitions arising from temperature-dependent phonon softening and anharmonic effects on dielectric properties remain poorly explored. In this study, we employ density-functional theory (DFT) and self-consistent phonon (SCP) calculations to investigate finite-temperature phonons in cubic niobate perovskites. To include explicit anharmonic vibrational effects, SCP frequencies are shifted by the bubble self-energy correction within the quasiparticle (QP) approximation, providing precise descriptions of phonon softening in these strongly anharmonic solids. We further calculate the static dielectric constant of KNbO3 and NaNbO3 as a function of temperature using the Lyddane-Sachs-Teller (LST) relation and QP-corrected phonon dispersions. Our theoretical results align with experimental data, offering reliable temperature-dependent phonon dispersions while considering anharmonic self-energies and thermal expansion effects, enhancing our understanding of the complex relations between lattice vibrations and phase transitions in these anharmonic oxides.

Isotope Effects in the Zundel-Eigen Isomerization of H+(H2O)6.

The isomerization pathway between the energetically low-lying Zundel and Eigen isomers of the protonated water hexamer was investigated using high-level ab initio calculations including a treatment of zero-point corrections. On the basis of these calculations, the Zundel-Eigen isomerization was found to proceed through a stable intermediate isomer, which consists of a four-membered ring with two single acceptor water molecules. The inclusion of vibrational zero-point energy is shown to be important for accurately establishing the relative energies of the three relevant isomers involved in the Zundel-Eigen isomerization. Diffusion Monte Carlo calculations including anharmonic vibrational effects show that all three isomers of H+(H2O)6 and D+(D2O)6 have well-defined structures. The energetic ordering of the three isomers changes upon deuteration. The implications of these results for the vibrational spectra of H+(H2O)6 and D+(D2O)6 are also discussed.

High-accuracy thermodynamic properties to the melting point from ab initio calculations aided by machine-learning potentials

Accurate prediction of thermodynamic properties requires an extremely accurate representation of the free-energy surface. Requirements are twofold—first, the inclusion of the relevant finite-temperature mechanisms, and second, a dense volume–temperature grid on which the calculations are performed. A systematic workflow for such calculations requires computational efficiency and reliability, and has not been available within an ab initio framework so far. Here, we elucidate such a framework involving direct upsampling, thermodynamic integration and machine-learning potentials, allowing us to incorporate, in particular, the full effect of anharmonic vibrations. The improved methodology has a five-times speed-up compared to state-of-the-art methods. We calculate equilibrium thermodynamic properties up to the melting point for bcc Nb, magnetic fcc Ni, fcc Al, and hcp Mg, and find remarkable agreement with experimental data. A strong impact of anharmonicity is observed specifically for Nb. The introduced procedure paves the way for the development of ab initio thermodynamic databases.

Quantum Monte Carlo study of three-dimensional Coulomb complexes: Trions and biexcitons, hydrogen molecules and ions, helium hydride cations, and positronic and muonic complexes

Three-dimensional (3D) excitonic complexes influence the optoelectronic properties of bulk semiconductors. More generally, correlated few-particle molecules and ions, held together by pairwise Coulomb potentials, play a fundamental role in a variety of fields in physics and chemistry. Based on statistically exact diffusion quantum Monte Carlo calculations, we have studied excitonic three- and four-body complexes (trions and biexcitons) in bulk 3D semiconductors as well as a range of small molecules and ions in which the nuclei are treated as quantum particles on an equal footing with the electrons. We present interpolation formulas that predict the binding energies of these complexes either in bulk semiconductors or in free space. By evaluating pair distribution functions within quantum Monte Carlo simulations, we examine the importance of harmonic and anharmonic vibrational effects in small molecules.

Atomistic description of self-diffusion in molybdenum: A comparative theoretical study of non-Arrhenius behavior

According to experimental observations, the temperature dependence of self-diffusion coefficient in most body-centered cubic metals (bcc) exhibits non-Arrhenius behavior. The origin of this behavior is likely related to anharmonic vibrational effects at elevated temperatures. However, it is still debated whether anharmonicity affects more the formation or migration of monovacancies, which are known to govern the self-diffusion. In this extensive atomistic simulation study we investigated thermodynamic properties of monovacancies in bcc molybdenum, here taken as a representative model system, from zero temperature to the melting point. We combined first-principles calculations and classical simulations based on three widely used interatomic potentials for Mo. In our analysis we employ static and dynamic atomistic calculations as well as statistical sampling techniques and thermodynamic integration to achieve thorough information about temperature variations of vacancy formation and migration free energies and diffusivities. In addition, we carry out large-scale molecular dynamics simulations that enable direct observation of high-temperature self-diffusion at the atomic scale. By scrutinizing the results obtained by different models and methods, we conclude that the peculiar self-diffusion behavior is likely caused by strong temperature dependence of the vacancy formation energy.

Theoretical study of the low-lying electronic states of iron hydride cation.

Both FeH and FeH+ are predicted to be abundant in cool stellar atmospheres and proposed to be molecular components of the gas phase interstellar medium (ISM). However, experimental and simulated data for both species are lacking, which have hindered astronomical detection. There are no published laboratory data for the spectroscopy of FeH+ in any frequency regime. It is also not established if FeH+ possesses salient multireference character, which would pose significant challenges for ab initio modeling of geometric and spectroscopic properties. With a set of high-level coupled cluster and multireference configuration interaction computations, a study of the electronic structure of the ground state and seven excited states of FeH+ was carried out. An X5Δi electronic ground state of FeH+ is found, in agreement with previous theoretical studies. Including corrections for spin-orbit coupling and anharmonic vibrational effects, the Ω = 3, ν = 0 spin ladder of the A5Πi electronic state lies 872 cm-1 higher in energy than the Ω = 4, ν = 0 spin ladder of the ground state. Combined with previous work in our laboratory, the ionization energy of FeH is computed to be 7.4851 eV. With modern multireference configuration interaction and coupled cluster methods, spectroscopic constants (re, Be, ωe, ωexe, αe, and D¯e) for several bound excited states (A5Πi, B 5Σi +, a 3Σr -, b3Φi, c3Πi, d3Δr, and 7Σ+) were characterized. This study will lead efforts to identify FeH+ in the ISM and help solve important remaining questions in quantifying metal-hydride bonding.

The high-resolution LIF spectrum of the SiCCl free radical: Probing the silicon-carbon triple bond

A rotationally resolved spectrum of a band of the A∼2Σ+ − X∼2Π electronic transition of the SiCCl free radical has been obtained for the first time. The radical was produced in an electric discharge jet using 1,1-dichlorosilacyclobutane vapor in high pressure argon as the precursor. The laser-induced fluorescence spectrum of the band system in the 610–550 nm region was recorded and the 2Π3/2 spin component of the 0–0 band was studied at high resolution. Rotational analysis gave the B values for the combining states and by fixing the CCl bond lengths at ab initio values we obtained r″ (SiC) = 1.692(1) Å and r' (SiC) = 1.594(1) Å. The bond lengths correspond to a silicon-carbon double bond in the ground state and an unusual SiC triple bond in the excited state. Single vibronic level emission spectra yielded the ground state bending and stretching energy levels up to energies near 5000 cm−1. These were satisfactorily fitted to a Renner-Teller model that included spin–orbit and vibrational anharmonicity effects.

Quantifying Anharmonic Vibrations in Thermoelectric Layered Cobaltites and Their Role in Suppressing Thermal Conductivity

Optimizing multiple materials properties which are simultaneously in competition with each other is one of the chief challenges in thermoelectric materials research. Introducing greater anharmonicity to vibrational modes is one strategy for suppressing phonon thermal transport in crystalline oxides without detrimentally affecting electronic conductivity, so that the overall thermoelectric efficiency can be improved. Based on perturbed molecular dynamics and associated numerical analyses, we show that CoO2 layers in layered cobaltite thermoelectrics NaxCoO2 and Ca3Co4O9 are responsible for most of the in-plane heat transport in these materials, and that the non-conducting intermediate layers in the two materials exhibit different kinds of anharmonicity. More importantly, thermal conduction is shown to be altered by modifying the structure of the intermediate layers. The simulation methods developed to quantify the effect of anharmonic atomic vibrations on thermal conductivity provide a new tool for the rational design of thermoelectric materials, and the insights gained should hasten the attainment of higher conversion efficiencies so that thermoelectrics can be put to widespread practical use.

Estimating the intrinsic limit of the Feller-Peterson-Dixon composite approach when applied to adiabatic ionization potentials in atoms and small molecules

Benchmark adiabatic ionization potentials were obtained with the Feller-Peterson-Dixon (FPD) theoretical method for a collection of 48 atoms and small molecules. In previous studies, the FPD method demonstrated an ability to predict atomization energies (heats of formation) and electron affinities well within a 95% confidence level of ±1 kcal/mol. Large 1-particle expansions involving correlation consistent basis sets (up to aug-cc-pV8Z in many cases and aug-cc-pV9Z for some atoms) were chosen for the valence CCSD(T) starting point calculations. Despite their cost, these large basis sets were chosen in order to help minimize the residual basis set truncation error and reduce dependence on approximate basis set limit extrapolation formulas. The complementary n-particle expansion included higher order CCSDT, CCSDTQ, or CCSDTQ5 (coupled cluster theory with iterative triple, quadruple, and quintuple excitations) corrections. For all of the chemical systems examined here, it was also possible to either perform explicit full configuration interaction (CI) calculations or to otherwise estimate the full CI limit. Additionally, corrections associated with core/valence correlation, scalar relativity, anharmonic zero point vibrational energies, non-adiabatic effects, and other minor factors were considered. The root mean square deviation with respect to experiment for the ionization potentials was 0.21 kcal/mol (0.009 eV). The corresponding level of agreement for molecular enthalpies of formation was 0.37 kcal/mol and for electron affinities 0.20 kcal/mol. Similar good agreement with experiment was found in the case of molecular structures and harmonic frequencies. Overall, the combination of energetic, structural, and vibrational data (655 comparisons) reflects the consistent ability of the FPD method to achieve close agreement with experiment for small molecules using the level of theory applied in this study.

Quantum Monte Carlo study of the energetics of the rutile, anatase, brookite, and columbiteTiO2polymorphs

The relative energies of the low-pressure rutile, anatase, and brookite polymorphs and the high-pressure columbite polymorph of TiO$_2$ have been calculated as a function of temperature using the diffusion quantum Monte Carlo (DMC) method and density functional theory (DFT). The vibrational energies are found to be important on the scale of interest and significant quartic anharmonicity is found in the rutile phase. Static-lattice DFT calculations predict that anatase is lower in energy than rutile, in disagreement with experiment. The accurate description of electronic correlations afforded by DMC calculations and the inclusion of anharmonic vibrational effects contribute to stabilizing rutile with respect to anatase. Our calculations predict a phase transition from anatase to rutile TiO$_2$ at $630 \pm 210$ K.

Effects of vibrational anharmonicity and inter-mode couplings on the binding energy of a positron to molecules

We theoretically analyzed the effect of the anharmonicity and the inter-mode couplings of molecular vibrations on a positron affinity (PA), which is the binding energy of a positron, of HCN and CH2O molecules. Based on the different theoretical approaches for describing vibrational wave functions, we confirmed that (i) not only the anharmonicity but also the inter-mode couplings on potential energy surface are indispensable for the accurate calculation of PA values, and (ii) the effect of inter-mode couplings on a positron affinity surface is sufficiently weaker than that on potential energy surface.

Anharmonic vibrational effects in linear and two-dimensional electronic spectra.

For the description of vibrational effects in electronic spectra, harmonic vibrations are a convenient and widespread model. However, spectra of larger organic molecules in solution usually exhibit signs of vibrational anharmonicity, as revealed by deviation from the mirror image symmetry between linear absorption and emission spectra of the harmonic case. For perylene and terylene, two molecules with rigid Pi-electron systems and strong vibrational-electronic coupling, we employ a simple but effective theoretical model, which introduces cubic anharmonicity in the potentials of electronic surfaces. Vibrational anharmonicity is then readily quantified based on the experimentally measured peak ratio of the first vibronic progression peaks in linear absorption and emission. This method is straightforward but not applicable if emission from the initially excited state is short lived. For such a case, we employ two-dimensional electronic spectroscopy in the visible as a comprehensive time-resolved technique for the experimental determination of the vibrational anharmonicity of pinacyanol iodide, a solvated dye molecule exhibiting ultrafast excited state isomerization. We show that the ratio between certain cross peak amplitudes in two-dimensional electronic spectra is a direct measure of vibrational anharmonicity.

Effects of vibrational anharmonicity on molecular electronic conduction and thermoelectric efficiency

We study inelastic vibration-assisted charge transfer effects in two-site molecular junctions, focusing on signatures of vibrational anharmonicity on the electrical characteristics and the thermoelectric response of the junction. We consider three types of oscillators: harmonic, anharmonic-Morse allowing bond dissociation, and harmonic-quartic, mimicking a confinement potential. Using a quantum master equation method which is perturbative in the electron-vibration interaction, we find that the (inelastic) electrical and thermal conductances can be largely affected by the nature of the vibrational potential. In contrast, the Seebeck coefficient, the thermoelectric figure-of-merit, and the thermoelectric efficiency beyond linear response conceal this information, showing a rather weak sensitivity to vibrational anharmonicity. Our work illustrates that anharmonic (many-body) effects, consequential to the current-voltage characteristics, are of little effect for the thermoelectric performance in the present model.

Temperature effects in first-principles solid state calculations of the chemical shielding tensor made simple.

We study the effects of atomic vibrations on the solid-state chemical shielding tensor using first principles density functional theory calculations. At the harmonic level, we use a Monte Carlo method and a perturbative expansion. The Monte Carlo method is accurate but computationally expensive, while the perturbative method is computationally more efficient, but approximate. We find excellent agreement between the two methods for both the isotropic shift and the shielding anisotropy. The effects of zero-point quantum mechanical nuclear motion are important up to relatively high temperatures: at 500 K they still represent about half of the overall vibrational contribution. We also investigate the effects of anharmonic vibrations, finding that their contribution to the zero-point correction to the chemical shielding tensor is small. We exemplify these ideas using magnesium oxide and the molecular crystals L-alanine and β-aspartyl-L-alanine. We therefore propose as the method of choice to incorporate the effects of temperature in solid state chemical shielding tensor calculations using the perturbative expansion within the harmonic approximation. This approach is accurate and requires a computational effort that is about an order of magnitude smaller than that of dynamical or Monte Carlo approaches, so these effects might be routinely accounted for.

Benchmarking ab initio binding energies of hydrogen-bonded molecular clusters based on FTIR spectroscopy.

Models of formation and growth of atmospheric aerosols are highly dependent on accurate cluster binding energies. These are most often calculated by ab initio electronic structure methods but remain associated with significant uncertainties. We present a computational benchmarking study of the Gibbs free binding energies in molecular complexes and clusters based on gas phase FTIR spectroscopy. The acetonitrile-HCl molecular complex is identified via its redshifted H-Cl stretching vibrational mode. We determine the Gibbs free binding energy, ΔG°295K, to between 4.8 and 7.9 kJ mol(-1) and compare this range to predictions from several widely used electronic structure methods, including five density functionals, Møller-Plesset perturbation theory, and five coupled cluster methods up to CCSDT quality, considering also the D3 dispersion correctional scheme. With some exceptions, we find that most electronic structure methods overestimate ΔG°295K. The effects of vibrational anharmonicity is approximated using scaling factors, reducing ΔG°295K by ca. 1.8 kJ mol(-1), whereby ΔG°295K predictions well within the experimental range can be obtained.

Toward Accurate and Efficient Predictions of Entropy and Gibbs Free Energy of Adsorption of High Nitrogen Compounds on Carbonaceous Materials

The adsorption of high nitrogen compounds (HNCs) on the selected adsorption sites of carbonaceous materials from the gas phase has been investigated by ab initio quantum chemical methods at the density functional level applying both periodic and cluster approaches with M06-2X and BLYP functionals including dispersion forces (BLYP-D2). Among the possible structures of the adsorption complexes, the most stable systems possess nitrogen-containing heterocycles in a parallel orientation toward the modeled carbon surface. The adsorption enthalpies, calculated using the rigid rotor-harmonic oscillator approach (RRHO), were in good agreement with available experimental data. This approach was shown to provide sufficiently accurate adsorption enthalpies from the gas phase for the HNC–carbon systems. The vibrational, rotational, and translation contributions to the adsorption entropy were also analyzed by the approach extended beyond the RRHO scheme. The effects of anharmonic vibrations and internal rotations of th...

On the role of disorder in catalysis driven by discrete breathers

A new mechanism of catalysis is discussed, which is based on the rate-promoting effect of large-amplitude anharmonic lattice vibrations, a.k.a. intrinsic localized modes or ‘discrete breathers’ (DBs), which can excite atoms at specific ‘active sites’ rather strongly, giving them energy far exceeding the energy of thermal vibrations for hundreds of oscillation periods. The DB-induced modulation of activation energies (free energy barriers between reactants and products) results in a drastic amplification of the reaction rates, which can be described by a simple analytical expression in the adiabatic limit. The striking site selectiveness of DB excitation dynamics in the presence of spatial (quenched) disorder makes these nonlinear vibrations viable candidates to play the role of ‘active modes’ in the catalytic process in various physical, chemical and biological systems.

Spectrum of vibrational frequencies of crystalline tungsten at temperatures of 293 and 2400 K

The slow neutron inelastic scattering spectra for a refractory (Tmelt = 3680 K) Group VI transition metal of the Periodic Table, namely, tungsten, were measured for the first time in the range from room temperature to 2400 K. Measurements of the neutron scattering spectra of tungsten were performed on a DIN-2PI time-of-flight spectrometer installed at the IBR-2 reactor (Dubna, Russia). The sample was heated in a TS3000 K high-temperature thermostat. The spectrum of vibrational frequencies of the crystal lattice of tungsten at temperatures of 293 and 2400 K was determined from the measured neutron scattering spectra by the iterative method. A softening of the frequency spectrum of tungsten was observed with increasing temperature. This was explained by the increasing role of vibrational anharmonicity effects at high temperatures. The experimental results were compared with model calculations of the frequency spectrum of tungsten.