Showing papers on "Potential energy surface published in 2021"

Phase Diagram of a Deep Potential Water Model.

Abstract: Using the Deep Potential methodology, we construct a model that reproduces accurately the potential energy surface of the SCAN approximation of density functional theory for water, from low temperature and pressure to about 2400 K and 50 GPa, excluding the vapor stability region The computational efficiency of the model makes it possible to predict its phase diagram using molecular dynamics Satisfactory overall agreement with experimental results is obtained The fluid phases, molecular and ionic, and all the stable ice polymorphs, ordered and disordered, are predicted correctly, with the exception of ice III and XV that are stable in experiments, but metastable in the model The evolution of the atomic dynamics upon heating, as ice VII transforms first into ice VII^{''} and then into an ionic fluid, reveals that molecular dissociation and breaking of the ice rules coexist with strong covalent fluctuations, explaining why only partial ionization was inferred in experiments

Computational approaches to dissociative chemisorption on metals: towards chemical accuracy

Abstract: We review the state-of-the-art in the theory of dissociative chemisorption (DC) of small gas phase molecules on metal surfaces, which is important to modeling heterogeneous catalysis for practical reasons, and for achieving an understanding of the wealth of experimental information that exists for this topic, for fundamental reasons. We first give a quick overview of the experimental state of the field. Turning to the theory, we address the challenge that barrier heights (Eb, which are not observables) for DC on metals cannot yet be calculated with chemical accuracy, although embedded correlated wave function theory and diffusion Monte-Carlo are moving in this direction. For benchmarking, at present chemically accurate Eb can only be derived from dynamics calculations based on a semi-empirically derived density functional (DF), by computing a sticking curve and demonstrating that it is shifted from the curve measured in a supersonic beam experiment by no more than 1 kcal mol-1. The approach capable of delivering this accuracy is called the specific reaction parameter (SRP) approach to density functional theory (DFT). SRP-DFT relies on DFT and on dynamics calculations, which are most efficiently performed if a potential energy surface (PES) is available. We therefore present a brief review of the DFs that now exist, also considering their performance on databases for Eb for gas phase reactions and DC on metals, and for adsorption to metals. We also consider expressions for SRP-DFs and briefly discuss other electronic structure methods that have addressed the interaction of molecules with metal surfaces. An overview is presented of dynamical models, which make a distinction as to whether or not, and which dissipative channels are modeled, the dissipative channels being surface phonons and electronically non-adiabatic channels such as electron-hole pair excitation. We also discuss the dynamical methods that have been used, such as the quasi-classical trajectory method and quantum dynamical methods like the time-dependent wave packet method and the reaction path Hamiltonian method. Limits on the accuracy of these methods are discussed for DC of diatomic and polyatomic molecules on metal surfaces, paying particular attention to reduced dimensionality approximations that still have to be invoked in wave packet calculations on polyatomic molecules like CH4. We also address the accuracy of fitting methods, such as recent machine learning methods (like neural network methods) and the corrugation reducing procedure. In discussing the calculation of observables we emphasize the importance of modeling the properties of the supersonic beams in simulating the sticking probability curves measured in the associated experiments. We show that chemically accurate barrier heights have now been extracted for DC in 11 molecule-metal surface systems, some of which form the most accurate core of the only existing database of Eb for DC reactions on metal surfaces (SBH10). The SRP-DFs (or candidate SRP-DFs) that have been derived show transferability in many cases, i.e., they have been shown also to yield chemically accurate Eb for chemically related systems. This can in principle be exploited in simulating rates of catalyzed reactions on nano-particles containing facets and edges, as SRP-DFs may be transferable among systems in which a molecule dissociates on low index and stepped surfaces of the same metal. In many instances SRP-DFs have allowed important conclusions regarding the mechanisms underlying observed experimental trends. An important recent observation is that SRP-DFT based on semi-local exchange DFs has so far only been successful for systems for which the difference of the metal work function and the molecule's electron affinity exceeds 7 eV. A main challenge to SRP-DFT is to extend its applicability to the other systems, which involve a range of important DC reactions of e.g. O2, H2O, NH3, CO2, and CH3OH. Recent calculations employing a PES based on a screened hybrid exchange functional suggest that the road to success may be based on using exchange functionals of this category.

Accurate Deep Potential model for the Al–Cu–Mg alloy in the full concentration space*

Abstract: Combining first-principles accuracy and empirical-potential efficiency for the description of the potential energy surface (PES) is the philosopher's stone for unraveling the nature of matter via atomistic simulation. This has been particularly challenging for multi-component alloy systems due to the complex and non-linear nature of the associated PES. In this work, we develop an accurate PES model for the Al-Cu-Mg system by employing Deep Potential (DP), a neural network based representation of the PES, and DP Generator (DP-GEN), a concurrent-learning scheme that generates a compact set of ab initio data for training. The resulting DP model gives predictions consistent with first-principles calculations for various binary and ternary systems on their fundamental energetic and mechanical properties, including formation energy, equilibrium volume, equation of state, interstitial energy, vacancy and surface formation energy, as well as elastic moduli. Extensive benchmark shows that the DP model is ready and will be useful for atomistic modeling of the Al-Cu-Mg system within the full range of concentration.

Full-dimensional potential energy surface for acetylacetone and tunneling splittings

Abstract: We present a full-dimensional potential energy surface for acetylacetone (AcAc) using full and fragmented permutationally invariant polynomial approaches. Previously reported MP2/aVTZ energies and gradients are augmented by additional calculations at this level of theory for the fits. Numerous stationary points are reported as are the usual metrics to assess the precision of the fit. The electronic barrier height for the H-atom transfer is roughly 2.2 kcal mol−1. Diffusion Monte Carlo (DMC) calculations are used to calculate the ground state wavefunction and zero-point energy of acetylacetone. These together with fixed-node DMC calculations for the first excited-state provide the predicted tunneling splitting due to the barrier to H-transfer separating two equivalent wells. Simpler 1d calculations of this splitting are also reported for varying barrier heights including the CCSD(T) barrier height of 3.2 kcal mol−1. Based on those results the DMC splitting of 160 cm−1 with a statistical uncertainty of roughly 21 cm−1, calculated using the MP2-based PES, is estimated to decrease to 100 cm−1 for a barrier of 3.2 kcal mol−1. The fragmented surface is shown to be fast to evaluate.

Establishment of a Reax force field to study SF 6 gas over-thermal decomposition

Abstract: This paper focuses on the problem of missing parameters in the Reax force field containing S and F elements. First, the density functional theory was used to scan SF6 and low-fluoride sulfide molecules to obtain the basic dataset for the Reax force field. Then Monte Carlo method was used to perform fitting optimization and quality verification on the established force field. On the basis of the established force field, molecular dynamics studies were carried out on the over-thermal decomposition of SF6 gas insulating medium. And the infrared spectra describing the vibration characteristics of SF6 and low-fluoride sulfide molecules were obtained. According to the vibration modes revealed by infrared spectra, the potential energy surface scan was performed.The paper focused on the analysis of the total energy, potential energy and kinetic energy of SF6 and low-fluoride sulfide molecules while moving at different temperatures. The results show that the energy error of the established force field is about 10% when describing the bonding and breaking processes of SF6, SF5, SF4, SF3 and SF2 molecules, which verifies the reliability of ReaxFF method to describe molecular behaviour. The research work in this paper can lay the foundation for the next systematic study of the microscopic physical mechanism of SF6 over-thermal decomposition.

Neural network potential energy surface for the low temperature ring polymer molecular dynamics of the H2CO + OH reaction.

Abstract: A new potential energy surface (PES) and dynamical study of the reactive process of H2CO + OH toward the formation of HCO + H2O and HCOOH + H are presented. In this work, a source of spurious long range interactions in symmetry adapted neural network (NN) schemes is identified, which prevents their direct application for low temperature dynamical studies. For this reason, a partition of the PES into a diabatic matrix plus a NN many-body term has been used, fitted with a novel artificial neural network scheme that prevents spurious asymptotic interactions. Quasi-classical trajectory (QCT) and ring polymer molecular dynamics (RPMD) studies have been carried on this PES to evaluate the rate constant temperature dependence for the different reactive processes, showing good agreement with the available experimental data. Of special interest is the analysis of the previously identified trapping mechanism in the RPMD study, which can be attributed to spurious resonances associated with excitations of the normal modes of the ring polymer.

Exploring the Chemical Space of Linear Alkane Pyrolysis via Deep Potential GENerator

Abstract: Reactive molecular dynamics (MD) simulation is a powerful tool to study the reaction mechanism of complex chemical systems. Central to the method is the potential energy surface (PES) that can desc...

Water Splitting by C60 -Supported Vanadium Single Atoms.

Abstract: Water splitting is an important source of hydrogen, a promising future carrier for clean and renewable energy. A detailed understanding of the mechanisms of water splitting, catalyzed by supported metal atoms or nanoparticles, is essential to improve the design of efficient catalysts. Here, we report an infrared spectroscopic study of such a water splitting process, assisted by a C60 supported vanadium atom, C60 V+ +H2 O→C60 VO+ +H2 . We probe both the entrance channel complex C60 V+ (H2 O) and the end product C60 VO+ , and observe the formation of H2 as a result from resonant infrared absorption. Density functional theory calculations exploring the detailed reaction pathway reveal that a quintet-to-triplet spin crossing facilitates the water splitting reaction by C60 -supported V+ , whereas this reaction is kinetically hindered on the isolated V+ ion by a high energy barrier. The C60 support has an important role in lowering the reaction barrier with more than 70 kJ mol-1 due to a large orbital overlap of one water hydrogen atom with one carbon atom of the C60 support. This fundamental insight in the water splitting reaction by a C60 -supported single vanadium atom showcases the importance of supports in single atom catalysts by modifying the reaction potential energy surface.

Data-driven magneto-elastic predictions with scalable classical spin-lattice dynamics

Abstract: A data-driven framework is presented for building magneto-elastic machine-learning interatomic potentials (ML-IAPs) for large-scale spin-lattice dynamics simulations. The magneto-elastic ML-IAPs are constructed by coupling a collective atomic spin model with an ML-IAP. Together they represent a potential energy surface from which the mechanical forces on the atoms and the precession dynamics of the atomic spins are computed. Both the atomic spin model and the ML-IAP are parametrized on data from first-principles calculations. We demonstrate the efficacy of our data-driven framework across magneto-structural phase transitions by generating a magneto-elastic ML-IAP for α-iron. The combined potential energy surface yields excellent agreement with first-principles magneto-elastic calculations and quantitative predictions of diverse materials properties including bulk modulus, magnetization, and specific heat across the ferromagnetic–paramagnetic phase transition.

Electrostatics, Charge Transfer, and the Nature of the Halide-Water Hydrogen Bond.

Abstract: Binary halide-water complexes X-(H2O) are examined by means of symmetry-adapted perturbation theory, using charge-constrained promolecular reference densities to extract a meaningful charge-transfer component from the induction energy. As is known, the X-(H2O) potential energy surface (for X = F, Cl, Br, or I) is characterized by symmetric left and right hydrogen bonds separated by a C2v-symmetric saddle point, with a tunneling barrier height that is <2 kcal/mol except in the case of F-(H2O). Our analysis demonstrates that the charge-transfer energy is correspondingly small (<2 kcal/mol except for X = F), considerably smaller than the electrostatic interaction energy. Nevertheless, charge transfer plays a crucial role determining the conformational preferences of X-(H2O) and provides a driving force for the formation of quasi-linear X··· H-O hydrogen bonds. Charge-transfer energies correlate well with measured O-H vibrational redshifts for the halide-water complexes and also for OH-(H2O) and NO2-(H2O), providing some indication of a general mechanism.

Evaluating the anharmonicity contributions to the molecular excited state internal conversion rates with finite temperature TD-DMRG

Abstract: In this work, we propose a new method to calculate molecular nonradiative electronic relaxation rates based on the numerically exact time-dependent density matrix renormalization group theory. This method could go beyond the existing frameworks under the harmonic approximation (HA) of the potential energy surface (PES) so that the anharmonic effect could be considered, which is of vital importance when the electronic energy gap is much larger than the vibrational frequency. We calculate the internal conversion (IC) rates in a two-mode model with Morse potential to investigate the validity of HA. We find that HA is unsatisfactory unless only the lowest several vibrational states of the lower electronic state are involved in the transition process when the adiabatic excitation energy is relatively low. As the excitation energy increases, HA first underestimates and then overestimates the IC rates when the excited state PES shifts toward the dissociative side of the ground state PES. On the contrary, HA slightly overestimates the IC rates when the excited state PES shifts toward the repulsive side. In both cases, a higher temperature enlarges the error of HA. As a real example to demonstrate the effectiveness and scalability of the method, we calculate the IC rates of azulene from S1 to S0 on the ab initio anharmonic PES approximated by the one-mode representation. The calculated IC rates of azulene under HA are consistent with the analytically exact results. The rates on the anharmonic PES are 30%–40% higher than the rates under HA.

Electric-field-induced wave-packet dynamics and geometrical rearrangement of trilobite Rydberg molecules

Abstract: We investigate the quantum dynamics of ultra-long-range trilobite molecules exposed to homogeneous electric fields. A trilobite molecule consists of a Rydberg atom and a ground-state atom, which is trapped at large internuclear distances in an oscillatory potential due to scattering of the Rydberg electron off the ground-state atom. Within the Born-Oppenheimer approximation, we derive an analytic expression for the two-dimensional adiabatic electronic potential energy surface in weak electric fields valid up to 500 V/m. This is used to unravel the molecular quantum dynamics employing the multiconfigurational time-dependent Hartree method. Quenches of the electric field are performed to trigger the wave-packet dynamics including the case of field inversion. Depending on the initial wave packet, we observe radial intrawell and interwell oscillations as well as angular oscillations and rotations of the respective one-body probability densities. Opportunities to control the molecular configuration are identified, a specific example being the possibility to superimpose different molecular bond lengths by a series of periodic quenches of the electric field.

Local energy decomposition analysis and molecular properties of encapsulated methane in fullerene (CH4@C60)

Abstract: Methane has been successfully encapsulated within cages of C60 fullerene, which is an appropriate model system to study confinement effects. Its chemistry and physics are also relevant for theoretical model descriptions. Here we provide insights into intermolecular interactions and predicted spectroscopic responses of the CH4@C60 complex and compared them with results from other methods and with data from the literature. Local energy decomposition analysis (LED) within the domain-based local pair natural orbital coupled cluster singles, doubles, and perturbative triples (DLPNO-CCSD(T)) framework was used, and an efficient protocol for studies of endohedral complexes of fullerenes is proposed. This approach allowed us to assess energies in relation to electronic and geometric preparation, electrostatics, exchange, and London dispersion for the CH4@C60 endohedral complex. The calculated stabilization energy of CH4 inside the C60 fullerene was −13.5 kcal mol−1 and its magnitude was significantly larger than the latent heat of evaporation of CH4. Evaluation of vibrational frequencies and polarizabilities of the CH4@C60 complex revealed that the infrared (IR) and Raman bands of the endohedral CH4 were essentially “silent” due to the dielectric screening effect of C60, which acted as a molecular Faraday cage. Absorption spectra in the UV-vis domain and ionization potentials of C60 and CH4@C60 were predicted. They were almost identical. The calculated 1H/13C NMR shifts and spin–spin coupling constants were in very good agreement with experimental data. In addition, reference DLPNO-CCSD(T) interaction energies for complexes with noble gases (Ng@C60; Ng = He, Ne, Ar, Kr) were calculated. The values were compared with those derived from supramolecular MP2/SCS-MP2 calculations and estimates with London-type formulas by Pyykko and coworkers [Phys. Chem. Chem. Phys., 2010, 12, 6187–6203], and with values derived from DFT-based symmetry-adapted perturbation theory (DFT-SAPT) by Hesselmann & Korona [Phys. Chem. Chem. Phys., 2011, 13, 732–743]. Selected points at the potential energy surface of the endohedral He2@C60 trimer were considered. In contrast to previous theoretical attempts with the DFT/MP2/SCS-MP2/DFT-SAPT methods, our calculations at the DLPNO-CCSD(T) level of theory predicted the He2@C60 trimer to be thermodynamically stable, which is in agreement with experimental observations.

Excited-State Properties and Relaxation Pathways of Selenium-Substituted Guanine Nucleobase in Aqueous Solution and DNA Duplex.

Abstract: The excited-state properties and relaxation mechanisms after light irradiation of 6-selenoguanine (6SeG) in water and in DNA have been investigated using a quantum mechanics/molecular mechanics (QM/MM) approach with the multistate complete active space second-order perturbation theory (MS-CASPT2) method. In both environments, the S11(nSeπ5*) and S21(πSeπ5*) states are predicted to be the spectroscopically dark and bright states, respectively. Two triplet states, T13(πSeπ5*) and T23(nSeπ5*), are found energetically below the S2 state. Extending the QM region to include the 6SeG-Cyt base pair slightly stabilizes the S2 state and destabilizes the S1, due to hydrogen-bonding interactions, but it does not affect the order of the states. The optimized minima, conical intersections, and singlet-triplet crossings are very similar in water and in DNA, so that the same general mechanism is found. Additionally, for each excited state geometry optimization in DNA, three kind of structures ("up", "down", and "central") are optimized which differ from each other by the orientation of the C═Se group with respect to the surrounding guanine and thymine nucleobases. After irradiation to the S2 state, 6SeG evolves to the S2 minimum, near to a S2/S1 conical intersection that allows for internal conversion to the S1 state. Linear interpolation in internal coordinates indicate that the "central" orientation is less favorable since extra energy is needed to surmount the high barrier in order to reach the S2/S1 conical intersection. From the S1 state, 6SeG can further decay to the T13(πSeπ5*) state via intersystem crossing, where it will be trapped due to the existence of a sizable energy barrier between the T1 minimum and the T1/S0 crossing point. Although this general S2 → T1 mechanism takes place in both media, the presence of DNA induces a steeper S2 potential energy surface, that it is expected to accelerate the S2 → S1 internal conversion.

Transferability of machine learning potentials: Protonated water neural network potential applied to the protonated water hexamer.

Abstract: A previously published neural network potential for the description of protonated water clusters up to the protonated water tetramer, H+(H2O)4, at an essentially converged coupled cluster accuracy [C. Schran, J. Behler, and D. Marx, J. Chem. Theory Comput. 16, 88 (2020)] is applied to the protonated water hexamer, H+(H2O)6—a system that the neural network has never seen before. Although being in the extrapolation regime, it is shown that the potential not only allows for quantum simulations from ultra-low temperatures ∼1 K up to 300 K but is also able to describe the new system very accurately compared to explicit coupled cluster calculations. This transferability of the model is rationalized by the similarity of the atomic environments encountered for the larger cluster compared to the environments in the training set of the model. Compared to the interpolation regime, the quality of the model is reduced by roughly one order of magnitude, but most of the difference to the coupled cluster reference comes from global shifts of the potential energy surface, while local energy fluctuations are well recovered. These results suggest that the application of neural network potentials in extrapolation regimes can provide useful results and might be more general than usually thought.

Vibrational mode-specificity in the dynamics of the Cl + C2H6 → HCl + C2H5 reaction.

Abstract: We report a detailed dynamics study on the mode-specificity of the Cl + C2H6 → HCl + C2H5 H-abstraction reaction. We perform quasi-classical trajectory simulations using a recently developed high-level ab initio full-dimensional potential energy surface by exciting five different vibrational modes of ethane at four collision energies. We find that all the studied vibrational excitations, except that of the CC-stretching mode, clearly promote the title reaction, and the vibrational enhancements are consistent with the predictions of the Sudden Vector Projection (SVP) model, with the largest effect caused by the CH-stretching excitations. Intramolecular vibrational redistribution is also monitored for the differently excited ethane molecule. Our results indicate that the mechanism of the reaction changes with increasing collision energy, with no mode-specificity at high energies. The initial translational energy mostly converts into product recoil, while a significant part of the excess vibrational energy remains in the ethyl radical. An interesting competition between translational and vibrational energies is observed for the HCl vibrational distribution: the effect of exciting the low-frequency ethane modes, having small SVP values, is suppressed by translational excitation, whereas a part of the excess vibrational energy pumped into the CH-stretching modes (larger SVP values) efficiently flows into the HCl vibration.

Energy exchange rate coefficients from vibrational inelastic O2(Σg-3) + O2(Σg-3) collisions on a new spin-averaged potential energy surface.

Abstract: A new spin-averaged potential energy surface (PES) for non-reactive O2(Σg−3) + O2(Σg−3) collisions is presented The potential is formulated analytically according to the nature of the principal interaction components, with the main van der Waals contribution described through the improved Lennard-Jones model All the parameters involved in the formulation, having a physical meaning, have been modulated in restricted variation ranges, exploiting a combined analysis of experimental and ab initio reference data The new PES is shown to be able to reproduce a wealth of different physical properties, ranging from the second virial coefficients to transport properties (shear viscosity and thermal conductivity) and rate coefficients for inelastic scattering collisions Rate coefficients for the vibrational inelastic processes of O2, including both vibration-to-vibration (V–V) and vibration-to-translation/rotation (V–T/R) energy exchanges, were then calculated on this PES using a mixed quantum–classical method The effective formulation of the potential and its combination with an efficient, yet accurate, nuclear dynamics treatment allowed for the determination of a large database of V–V and V–T/R energy transfer rate coefficients in a wide temperature range

Transition states, reaction paths, and thermochemistry using the nuclear-electronic orbital analytic Hessian.

Abstract: The nuclear–electronic orbital (NEO) method is a multicomponent quantum chemistry theory that describes electronic and nuclear quantum effects simultaneously while avoiding the Born–Oppenheimer approximation for certain nuclei Typically specified hydrogen nuclei are treated quantum mechanically at the same level as the electrons, and the NEO potential energy surface depends on the classical nuclear coordinates This approach includes nuclear quantum effects such as zero-point energy and nuclear delocalization directly into the potential energy surface An extended NEO potential energy surface depending on the expectation values of the quantum nuclei incorporates coupling between the quantum and classical nuclei Herein, theoretical methodology is developed to optimize and characterize stationary points on the standard or extended NEO potential energy surface, to generate the NEO minimum energy path from a transition state down to the corresponding reactant and product, and to compute thermochemical properties For this purpose, the analytic coordinate Hessian is developed and implemented at the NEO Hartree–Fock level of theory These NEO Hessians are used to study the SN2 reaction of ClCH3Cl− and the hydride transfer of C4H9+ For each system, analysis of the single imaginary mode at the transition state and the intrinsic reaction coordinate along the minimum energy path identifies the dominant nuclear motions driving the chemical reaction Visualization of the electronic and protonic orbitals along the minimum energy path illustrates the coupled electronic and protonic motions beyond the Born–Oppenheimer approximation This work provides the foundation for applying the NEO approach at various correlated levels of theory to a wide range of chemical reactions

Systematic cluster growth: a structure search method for transition metal clusters

Abstract: The systematic cluster growth (SCG) method is a biased structure search strategy based on a seeding process for investigating the structural evolution and growth pattern of transition metal clusters. In SCG, a set of initial structures with size n are constructed based on the equilibrium structures of the preceding n − 1 cluster isomers by adding a single atom at all inequivalent binding sites. This strategy requires a relatively low number of evaluations for global minima localization on the potential energy surface, allowing its application in first-principles calculations. The performance of SCG is tested by using the Lennard Jones (LJ) potential energy surface. The 93.7% of the best-known solutions for Lennard Jones clusters were found for n ≤ 80 by using a relatively low number of local optimizations. Most importantly, by using SCG combined with DFT calculations (SCG-DFT), we revisit and provide the ground state structures and growth pattern for transition metal clusters TMn (where TM = Ti, Ni, Cu, Ag, Pt; and n = 6–14). The application of the code for doped clusters is also discussed. A detailed description of the present method for generating the structures of the clusters is provided.

Combined Surface-Hopping, Dyson Orbital, and B-Spline Approach for the Computation of Time-Resolved Photoelectron Spectroscopy Signals: The Internal Conversion in Pyrazine.

Abstract: A computational protocol for simulating time-resolved photoelectron signals of medium-sized molecules is presented. The procedure is based on a trajectory surface-hopping description of the excited-state dynamics and a combined Dyson orbital and multicenter B-spline approach for the computation of cross sections and asymmetry parameters. The accuracy of the procedure has been illustrated for the case of ultrafast internal conversion of gas-phase pyrazine excited to the 1B2u(ππ*) state. The simulated spectra and the asymmetry map are compared to the experimental data, and a very good agreement was obtained without applying any energy-dependent rescaling or broadening. An interesting side result of this work is the finding that the signature of the 1Au(nπ*) state is indistinguishable from that of the 1B3u(nπ*) state in the time-resolved photoelectron spectrum. By locating four symmetrically equivalent minima on the lowest-excited (S1) adiabatic potential energy surface of pyrazine, we revealed the strong vibronic coupling of the 1Au(nπ*) and 1B3u(nπ*) states near the S1 ← S0 band origin.

Effects of van der Waals interactions on the structure and stability of Cu8-xPdx (x = 0, 4, 8) cluster isomers

Abstract: Detailed knowledge of the minimum energy and low-lying structures of metal clusters is crucial to understand their physicochemical properties. In this work, the effect of van der Waals (vdW) interactions on the structure and stability of Cu-Pd clusters is investigated. With this aim, a global search algorithm that combines the basin-hopping method and density functional theory is used to explore the potential energy surface of Cu8-xPdx (x = 0, 4, 8) clusters. We present the structural motifs adopted by the low-energy isomers obtained with the PBE gereralized gradient exchange-correlation functional, and with PBE supplemented by dispersion energy corrections as given by Grimme’s D3 method. The isomeric structures obtained by PBE and PBE-D3 methods are in general quite similar, except for minor changes in some particular isomers. Global structural explorations were also performed with the meta-GGA TPSS and the hybrid PBE0 functionals, showing a reasonable agreement with the results obtained with the PBE and corrected PBE-D3 functionals. The energetics of the clusters is investigated by analyzing the binding energy per atom. Binding ;1;energies become larger when vdW forces are included, and this contribution increases from Cu8 to Pd8 and then to Cu4Pd4. Mixing between Cu and Pd atoms to form nanoalloys is favorable, a feature consistent with the well-known formation of intermetallic compounds in the bulk Cu-Pd alloys. The HOMO-LUMO gap indicates that some isomers in Cu8 and Cu4Pd4 are more reactive than the lowest energy isomer, a feature that gives a hint into the importance that low-lying isomers may have in catalytic reactions. Binding energies and HOMO-LUMO gaps present a stronger dependence on the exchange-correlation functional than the cluster structures.

Deconstructing Vibrational Motions on the Potential Energy Surfaces of Hydrogen-Bonded Complexes

Abstract: Internal vibrations underlie transient structure formation, spectroscopy, and dynamics. However, at least two challenges exist when aiming to elucidate the contributions of vibrational motions on t...

First-Principles Reaction Dynamics beyond Six-Atom Systems.

Abstract: Moving beyond the six-atomic benchmark systems, we discuss the new age and future of first-principles reaction dynamics, which investigates complex, multichannel chemical reactions. We describe the methodology starting from the benchmark ab initio characterization of the stationary points, followed by full-dimensional potential energy surface (PES) developments and reaction dynamics computations. We highlight our composite ab initio approach providing benchmark stationary-point properties with subchemical accuracy, the Robosurfer program system enabling automatic PES development, and applications for the Cl + C2H6, F + C2H6, and OH- + CH3I post-six-atom reactions focusing on ab initio issues and their solutions as well as showing the excellent agreement between theory and experiment.

Uncovering an oxide ion substitution for the OH− + CH3F reaction

Abstract: Theoretical investigations on chemical reactions allow us to understand the dynamics of the possible pathways and identify new unexpected routes. Here, we develop a global analytical potential energy surface (PES) for the OH− + CH3F reaction in order to perform high-level dynamics simulations. Besides bimolecular nucleophilic substitution (SN2) and proton abstraction, our quasi-classical trajectory computations reveal a novel oxide ion substitution leading to the HF + CH3O− products. This exothermic reaction pathway occurs via the CH3OH⋯F− deep potential well of the SN2 product channel as a result of a proton abstraction from the hydroxyl group by the fluoride ion. The present detailed dynamics study of the OH− + CH3F reaction focusing on the surprising oxide ion substitution demonstrates how incomplete our knowledge is of fundamental chemical reactions.