Showing papers on "Ab initio quantum chemistry methods published in 2022"

Possible Applications of Mo2c in the Orthorhombic and Hexagonal Phases Explored Via Ab-Initio Investigations of Elastic, Bonding, Optoelectronic and Thermophysical Properties

Abstract: Binary carbides demonstrate attractive set of physical properties that are suitable for numerous and diverse applications. In the present study, we have explored the structural properties, electronic structures, elastic constants, acoustic behaviors, phonon dispersions, optical properties, and various thermophysical properties of binary orthoand hexa-Mo2C (O-MC and H-MC, respectively) compounds in details via first-principles calculations using the density functional theory (DFT). The calculated ground state lattice parameters in both the symmetries are in excellent agreement with available experimental results. The calculated electronic band structure, density of states, and optical properties of Mo2C in both structures reveal metallic features. The orthorhombic crystal shows higher level mechanical and thermal anisotropy compared to that in the hexagonal phase. The elastic constants and phonon dispersion calculations show that, in both structures, Mo2C is mechanically and dynamically stable. A comprehensive mechanical and thermophysical study shows that both phases possess high structural stability, reasonably good machinability, ductile nature, high hardness, low compressibility, high Debye temperature and high melting temperature. Moreover, the electronic energy density of states, electron density distribution, elastic properties, and Mulliken bond population analyses indicate that the structures under consideration consist of mixed bonding characteristics with ionic and covalent contributions. Investigation of the optical properties reveals that the reflectivity spectra are anisotropic with respect to the polarization directions of the electric field in the visible to mid‐ultraviolet regions. High reflectivity over wide spectral range makes the compound suitable as reflecting coating. Both the structures are efficient absorber of ultraviolet radiation. The refractive indices are quite high in the infrared to visible range. Both structures show directional (plane) optical anisotropy. Though, hexa-Mo2C exhibits stronger optical anisotropy than the ortho-Mo2C.

Magnetic couplings in edge-sharing high-spin d 7 compounds

Abstract: High-spin d 7 Co(II) compounds have recently been identified as possible platforms for realizing highly anisotropic and bond-dependent couplings featured in quantum-compass models such as the celebrated Kitaev model. In order to evaluate this potential, we consider all symmetry-allowed contributions to the magnetic exchange for ideal edge-sharing bonds. Though a combination of ab-initio and cluster many-body calculations we conclude that bond-dependent couplings are generally suppressed in favor of Heisenberg exchange for real materials. Consequences for several prominent materials including Na2Co2TeO6 and BaCo2(AsO4)2 are discussed.

HeH+ Collisions with H2: Rotationally Inelastic Cross Sections and Rate Coefficients from Quantum Dynamics at Interstellar Temperatures

Abstract: We report for the first time an accurate ab initio potential energy surface for the HeH+–H2 system in four dimensions (4D) treating both diatomic species as rigid rotors. The computed ab initio potential energy point values are fitted using an artificial neural network method and used in quantum close coupling calculations for different initial states of both rotors, in their ground electronic states, over a range of collision energies. The state-to-state cross section results are used to compute the rate coefficients over a range of temperatures relevant to interstellar conditions. By comparing the four dimensional quantum results with those obtained by a reduced-dimensions approach that treats the H2 molecule as an averaged, nonrotating target, it is shown that the reduced dimensionality results are in good accord with the four dimensional results as long as the HeH+ molecule is not initially rotationally excited. By further comparing the present rate coefficients with those for HeH+–H and for HeH+–He, we demonstrate that H2 molecules are the most effective collision partners in inducing rotational excitation in HeH+ cation at interstellar temperatures. The rotationally inelastic rates involving o-H2 and p-H2 excitations are also obtained and they turn out to be, as in previous systems, orders of magnitude smaller than those involving the cation. The results for the H2 molecular partner clearly indicate its large energy-transfer efficiency to the HeH+ system, thereby confirming its expected importance within the kinetics networks involving HeH+ in interstellar environments.

Improving Lattice Rigidity and Charge Carrier Lifetime by Engineering Spacer Cation of Ruddlesden-Popper Perovskites: A Time-Domain Ab Initio Study.

Abstract: First-principles quantum dynamics calculations show that charge carrier lifetimes, charge transport, and lattice stability are notably improved when BA (CH3(CH2)3NH3+) in BA2PbI4 is replaced with MTEA (CH3(CH2)2SNH3+). By suppressing atomic fluctuations, MTEA enhances the lattice stiffness and inhibits loss of coherence due to the S-S interaction. By delocalizing hole wave functions on the MTEA, particularly on the S atoms, while maintaining the electron wave functions largely unchanged compared to the BA2PbI4, MTEA serves to enhance charge transport and NA coupling while narrowing the bandgap by 0.18 eV. Overall, MTEA decreases NA coupling due to slow atomic motions against a large overlap of electron-hole wave functions, which suppresses nonradiative electron-hole recombination and prolongs carrier lifetime twice longer compared with BA2PbI4. This simulation presents a rational route to make high performance two-dimensional perovskite solar cells.

Discovery of a new molecular ion, HC7NH+, in TMC-1

Abstract: We report the detection of the protonated form of HC7N in TMC-1. The discovery of the cation HC7NH was carried out via the observation of nine harmonically related lines in the Q-band using the Yebes 40 m radiotelescope. The observed frequencies allowed us to obtain the rotational constants B0 = 553.938802 (160) MHz and D0 = 3.6292 (705) Hz. The identification of HC7NH is further supported by accurate ab initio calculations. We derived a column density of (5.5± 0.7)× 1010 cm−2, which constitutes another piece of evidence for the identification of the carrier. In addition, we revised the HC7N column density and we derived a new value of (2.1± 0.2)× 1013 cm−2. Hence, the abundance ratio HC7N/HC7NH is ∼380, while those for HC3N/HC3NH and HC5N/HC5NH are ∼230 and ∼240, respectively. Here, we discuss these results within the framework of a chemical model for protonated molecules in cold dense clouds.

Accurate <i>ab initio</i> calculations of RaF electronic structure appeal to more laser-spectroscopical measurements

Abstract: Recently, a breakthrough has been achieved in laser-spectroscopic studies of short-lived radioactive compounds with the first measurements of the radium monofluoride molecule (RaF) UV/vis spectra. We report results from high-accuracy ab initio calculations of the RaF electronic structure for ground and low-lying excited electronic states. Two different methods agree excellently with experimental excitation energies from the electronic ground state to the 2Π1/2 and 2Π3/2 states, but lead consistently and unambiguously to deviations from experimental-based adiabatic transition energy estimates for the 2Σ1/2 excited electronic state, and show that more measurements are needed to clarify spectroscopic assignment of the 2Δ state.

A full-dimensional ab initio intermolecular potential energy surface and dipole moment surfaces for H2O–Ar

Abstract: The H2O–Ar system has attracted broad interest in recent years because it is an important model to study inelastic scattering between atoms and triatomic molecules. A high-accuracy intermolecular potential energy surface (IPES) is the foundation for theoretical study on molecular collision dynamics for H2O–Ar. In addition, dipole moment surfaces (DMSs) are one of the prerequisites for spectral simulation. This study aims to obtain a full-dimensional intermolecular potential energy surface and dipole moment surfaces for the van der Waals complex H2O–Ar. In this study, ab initio energy points were computed at the frozen-core (FC) explicitly correlated coupled cluster [FC-CCSD(T)-F12a] level, with the augmented correlation-consistent polarized valence quadruple-zeta basis set plus bond functions. The permutation invariant polynomial neural network (PIP-NN) approach is adopted to fit the IPES, while the DMSs are constructed at the MP2/AVTZ level and fitted by the NN approach. With a root-mean-square-error (RMSE) of 0.284 cm-1, the IPES can accurately describe the motion of the H2O–Ar complex between R = 4 and 20 a0 in the energy range up to 10000 cm-1. The fitting errors of all the data points are 6.192 and 6.509 mDebye for the X and Z components, respectively. The global minimum of 140.633 cm-1 has the plane geometry, while the dipole moment of H2O–Ar is 1.853 Debye at the equilibrium structure. In summary, we report a full-dimensional intermolecular potential energy surface for H2O–Ar. These potentials precisely fit to CCSD(T)-F12a electronic energies with large basis set. The corresponding dipole moment surfaces have also been reported. In comparison with previous work, the employment of the high-level ab initio method will make our IPES more reliable. Several typical 2D contour plots of the IPES and DMSs are also shown. The argon atom has a weak effect on the dipole moment of the H2O–Ar complex. The FORTRAN codes to generate 6D potentials and dipole moments reported here are available on request from the authors.

Ab initio surface free energies of tungsten with full account of thermal excitations

Abstract: The surface free energies of seven different facets of tungsten (W) are obtained up to the melting point with full account of all the relevant thermal excitations; in particular, thermal atomic vibrations, electronic excitations, and their mutual coupling. The latter is done using ab initio molecular dynamics simulations coupled with the thermodynamic integration technique. In this way, the calculations contain almost no error but the one related to the used exchange-correlation functional, which makes the results truly first principles. The obtained results are compared with previous quasiharmonic calculations for the surface free energies of W and experimental data. The anharmonic contribution is, as expected, important for open surfaces at high temperatures, which leads to a temperature dependence of the surface energy anisotropy. The calculated Wulff shapes and surface energies are in excellent agreement with experimental data close to the melting point, where the crystalline structure of the surface layers is destroyed by a dramatic mobility of the atoms there.

Non-Hermitian cavity quantum electrodynamics–configuration interaction singles approach for polaritonic structure with <i>ab initio</i> molecular Hamiltonians

Abstract: We combine ab initio molecular electronic Hamiltonians with a cavity quantum electrodynamics model for dissipative photonic modes and apply mean-field theories to the ground- and excited-states of resulting polaritonic systems. In particular, we develop a non-Hermitian configuration interaction singles theory for mean-field ground- and excited-states of the molecular system strongly interacting with a photonic mode and apply these methods to elucidating the phenomenology of paradigmatic polaritonic systems. We leverage the Psi4Numpy framework to yield open-source and accessible reference implementations of these methods.

Experimental and theoretical insights into Co-Ln magnetic exchange and the rare slow-magnetic relaxation behavior of [CoII2Pr]2+ in a series of linear [CoII2Ln]2+ complexes.

Abstract: We herein report a series of near-linear trinuclear complexes [Co2Ln(HL)4(NO3)](NO3)2 (where HL = (2-methoxy-6-[(E)-2'-hydroxymethyl-phenyliminomethyl]-phenolate) with Ln(III) = La (1), Ce (2), Pr (3)). For the comparative study, we have also included the recently reported analogous complexes of Gd(III), Tb(III), and Dy(III) (complexes 4-6) with the same H2L ligand. The experimental nature of the dc magnetic susceptibilities profile and an empirical approach revealed that the magnetic exchange interaction between Co(II) and Ln(III) having <4f7 (complexes 2 and 3) is antiferromagnetic while the dominant interaction between Co(II) and Ln(III) having ≥4f7 (complexes 4-6) is ferromagnetic. Dynamic magnetic relaxation studies on complexes 1-3 revealed the field induced single-molecule magnetic (SMM) behavior of 1 and 3 with effective energy barriers of 10.65 K and 15.03 K respectively, for magnetic relaxation. To the best of our knowledge, 3d-Pr(III) based zero or field induced SMMs have not been reported to date. CASSCF/SO-RASSI/SINGLE_ANISO based ab initio calculations on the X-ray structures of complexes 1-6, followed by POLY_ANISO simulations, estimated the magnetic exchange coupling constants JCo-Ln and JCo-Co and also rationalized our experimental findings for the dynamic magnetic properties.

Rational approach for an optimized formulation of silver-exchanged zeolites for iodine capture from first-principles calculations

Abstract: Ab initio calculations have been carried out to investigate in detail the effect of potential inhibiting species (CO, H2O, CH3Cl and Cl2) on the adsorption of iodine species (I2 and CH3I) in silver-exchanged zeolites of different Si/Al ratios and structures.

<i>Ab initio</i> calculations in atoms, molecules, and solids, treating spin–orbit coupling and electron interaction on an equal footing

Abstract: We incorporate explicit, non-perturbative treatment of spin-orbit coupling into ab initio auxiliary-field quantum Monte Carlo (AFQMC) calculations. The approach allows a general computational framework for molecular and bulk systems in which material specificity, electron correlation, and spin-orbit coupling effects can be captured accurately and on an equal footing, with favorable computational scaling vs system size. We adopt relativistic effective-core potentials that have been obtained by fitting to fully relativistic data and that have demonstrated a high degree of reliability and transferability in molecular systems. This results in a two-component spin-coupled Hamiltonian, which is then treated by generalizing the ab initio AFQMC approach. We demonstrate the method by computing the electron affinity in Pb, the bond dissociation energy in Br2 and I2, and solid Bi.

Revealing the exotic structure of molecules in strong magnetic fields.

Abstract: A novel implementation for the calculation of molecular gradients under strong magnetic fields is employed at the current-density functional theory level to optimize the geometries of molecular structures, which change significantly under these conditions. An analog of the ab initio random structure search is utilized to determine the ground-state equilibrium geometries for Hen and CHn systems at high magnetic field strengths, revealing the most stable structures to be those in high-spin states with a planar geometry aligned perpendicular to the field. The electron and current densities for these systems have also been investigated to develop an explanation of chemical bonding in the strong field regime, providing an insight into the exotic chemistry present in these extreme environments.

Structural, magnetic, and electronic properties of EuSi2 thin films on the Si(111) surface.

Abstract: Searching for magnetic silicide thin films has long been a hot topic in condensed matter physics and materials science based on their fundamental physics and promising device applications. Here we report a systematic study on the structural, magnetic, and electronic properties of EuSi2 thin films on the Si(111) surface by ab initio calculations. Total energy calculations show that the EuSi2 thin film in AA stacking is more favorable than that in AB or ABC stacking. The Eu2 + ions are coupled ferromagnetically within each layer and antiferromagnetically across the adjacent silicene layers with a large local spin moment of 6.96-7.00μB derived from the Eu-4f orbital electrons. Electronic band structure calculations indicate that the monolayer EuSi2 thin film is a semiconductor with an indirect surface band gap of 0.45 eV, while the multilayer EuSi2 thin films exhibit metallic behavior. These findings provide a systematic understanding of rare-earth metal silicides on the Si surface and will provide guidance for Si-based nanoelectronics and spintronics.

Origin of the herringbone reconstruction of Au(111) surface at the atomic scale

Abstract: The origin of the herringbone reconstruction on Au(111) surface has never been explained properly at the atomic level because the large periodic length (~30 nm) does not allow ab initio simulations of the system and because of the lack of highly accurate empirical force field. We trained a machine learning force field with high accuracy to explore this reconstruction. Our study shows that the lattice deformation in Au deeper layers, which allows the effective relaxation of the densified and anisotropic top layer lattice, is critical for the herringbone reconstruction. The herringbone reconstruction is energetically more favorable than the stripe reconstruction only if the slab thickness exceeds 12 atomic layers. Furthermore, we reveal the high stability of herringbone reconstruction at high temperatures and that a slight strain of about ±0.2% can induce a transition from the herringbone pattern to the stripe pattern, and both agree well with the experimental observations.

Ab Initio Molecular Dynamics of Temporary Anions Using Complex Absorbing Potentials

Abstract: Dissociative electron attachment, that is, the cleavage of chemical bonds induced by low-energy electrons, is difficult to model with standard quantum-chemical methods because the involved anions are not bound but subject to autodetachment. We present here a new computational development for simulating the dynamics of temporary anions on complex-valued potential energy surfaces. The imaginary part of these surfaces describes electron loss, whereas the gradient of the real part represents the force on the nuclei. In our method, the forces are computed analytically based on Hartree–Fock theory with a complex absorbing potential. Ab initio molecular dynamics simulations for the temporary anions of dinitrogen, ethylene, chloroethane, and the five mono- to tetrachlorinated ethylenes show qualitative agreement with experiments and offer mechanistic insights into dissociative electron attachments. The results also demonstrate how our method evenhandedly deals with molecules that may undergo dissociation upon electron attachment and those which only undergo autodetachment.

Understanding the Temperature Dependence and Finite Size Effects in Ab Initio MD Simulations of the Hydrated Electron.

Abstract: The hydrated electron is of interest to both theorists and experimentalists as a paradigm solution-phase quantum system. Although the bulk of the theoretical work studying the hydrated electron is based on mixed quantum/classical (MQC) methods, recent advances in computer power have allowed several attempts to study this object using ab initio methods. The difficulty with employing ab initio methods for this system is that even with relatively inexpensive quantum chemistry methods such as density functional theory (DFT), such calculations are still limited to at most a few tens of water molecules and only a few picoseconds duration, leaving open the question as to whether the calculations are converged with respect to either system size or dynamical fluctuations. Moreover, the ab initio simulations of the hydrated electron that have been published to date have provided only limited analysis. Most works calculate the electron's vertical detachment energy, which can be compared to experiment, and occasionally the electronic absorption spectrum is also computed. Structural features, such as pair distribution functions, are rare in the literature, with the majority of the structural analysis being simple statements that the electron resides in a cavity, which are often based only on a small number of simulation snapshots. Importantly, there has been no ab initio work examining the temperature-dependent behavior of the hydrated electron, which has not been satisfactorily explained by MQC simulations. In this work, we attempt to remedy this situation by running DFT-based ab initio simulations of the hydrated electron as a function of both box size and temperature. We show that the calculated properties of the hydrated electron are not converged even with simulation sizes up to 128 water molecules and durations of several tens of picoseconds. The simulations show significant changes in the water coordination and solvation structure with box size. Our temperature-dependent simulations predict a red-shift of the absorption spectrum (computed using TD-DFT with an optimally tuned range-separated hybrid functional) with increasing temperature, but the magnitude of the predicted red-shift is larger than that observed experimentally, and the absolute position of the calculated spectra are off by over half an eV. The spectral red-shift at high temperatures is accompanied by both a partial loss of structure of the electron's central cavity and an increased radius of gyration that pushes electron density onto and beyond the first solvation shell. Overall, although ab initio simulations can provide some insights into the temperature-dependent behavior of the hydrated electron, the simulation sizes and level of quantum chemistry theory that are currently accessible are inadequate for correctly describing the experimental properties of this fascinating object.

Effect of the cation structure on the properties of homobaric imidazolium ionic liquids.

Abstract: In this work we investigate the structure-property relationships in a series of alkylimidazolium ionic liquids with almost identical molecular weight. Using a combination of theoretical calculations and experimental measurements, we have shown that re-arranging the alkyl side chain or adding functional groups results in quite distinct features in the resultant ILs. The synthesised ILs, although structurally very similar, cover a wide spectrum of properties ranging from highly fluid, glass forming liquids to high melting point crystalline salts. Theoretical ab initio calculations provide insight on minimum energy orientations for the cations, which then are compared to experimental X-ray crystallography measurements to extract information on hydrogen bonding and to verify our understanding of the studied structures. Molecular dynamics simulations of the simplest (core) ionic liquids are used in order to help us interpret our experimental results and understand better why methylation of C2 position of the imidazolium ring results in ILs with such different properties compared to their non-methylated analogues.