Many-body Potential Energy Surface Research Articles

Hydrogen recombination on a mixed adsorption layer at saturation on a metal surface: H → (D + H) sat + Ni( [formula omitted

Interactions of H atom beams with (D + H)-covered Ni(1 0 0) surfaces are simulated at saturation level of 0.93 monolayer using quasi-classical microcanonical trajectory method. The Ni substrate is treated as a non-rigid multilayer slab using an embedded-atom method. The model many-body potential energy surface London–Eyring–Polanyi–Sato used to characterize the interactions between H–H and H–Ni(1 0 0) systems parameterized by fitting to the results of detailed total-energy calculations based on density functional theory. Since most of the incident H atoms trap to form hot atoms, reactions between the projectile atom and adsorbates are mainly due to the hot atom process. Results of a linear behavior of the total HD and quadratic behavior of the D 2 yields with the initial D coverage, in addition, significantly low secondary reactions between the adsorbates are found to be in good agreement with the experiment. In addition distributions of the rotational states of the product molecules, molecular desorption angles, vibrational states of the product molecules, molecular formation and desorption time, total and translational energies of the product molecules are also calculated as functions of different H and D coverages on the surface.

Theoretical approach to the laser-induced melting of graphite under different pressure conditions

We present a theoretical study of the laser-induced femtosecond melting of (1) graphite under high external pressure and (2) ultrathin graphite films under normal conditions. Our approach consists of molecular dynamic simulations performed on the basis of a time-dependent, many-body potential energy surface derived from a tight-binding Hamiltonian. Our results show that the laser-induced melting process occurs in two steps: (i) destruction of the graphite sheets via bond breaking, and (ii) merging of the melted layers. The separation of the two steps is more evident for graphite under pressure (10 GPa), but is also present in graphite films at normal pressure. The melting product is a low-density carbon phase, which remains stable under high pressure, but is unstable with an ultrashort life-time under normal pressure.

Ab initio, kinetics, and dynamics study of Cl+CH4→HCl+CH3

The Cl+CH4→HCl+CH3 reaction has been studied using different levels of the chemical reaction theory. Ab initio calculations at the fourth-order Møller–Plesset perturbation theory//second-order Møller–Plesset perturbation theory level, employing the 6-311G(2df,2pd) basis set, satisfactorily describe the system stationary points. A kinetics study using variational transition state theory has been accomplished, using ab initio information along the minimum energy reaction path. The agreement with experimental rate constants in the 200–500 K range is the best reported up to date. Reduced-dimensionality quasiclassical trajectory (QCT) calculations of the reaction dynamics have been performed on an analytical many-body potential energy surface, which is fitted to ab initio calculations of the system. The various experiments showing the absence of energy release to the CH3 group has led us to invoke a pseudotriatomic approximation, in which the CH3 moiety is treated as a single particle, so as to carry out dynamics calculations. Our QCT calculations give particular attention to the comparison with the numerous experiments available on this reaction. These include internal state distributions of the nascent diatomic molecule and state specific angular distributions. Further insight into the state specific scattering distributions is provided through analysis of opacity functions. The satisfactory reproduction of a great variety of experimental results by the theoretical study presented here is consistent with the idea that the CH3 modes are weakly coupled to the reaction coordinate, thus validating the model adopted in this work.

A New Semiempirical Approach to Study Ground and Excited States of Metal Complexes in Biological Systems

In this paper we develop a “diatomic in molecules semiempirical ligand field” (DIMSELF) method to calculate ground and excited many-body potential energy surfaces for an arbitrary transition metal ion in an arbitrary complex system. This method is not restricted to a high-symmetry environment and is meant to be inexpensive and suitable for nonadiabatic excited states dynamics on-the-fly. Within the approximations employed, the method includes full CI (configuration interaction) and SO (spin-orbit) interactions, essential to the description of nonradiative transitions such as those of myoglobin in the presence of carbon monoxide. We test our method against high level ab initio calculations for a simple model system of myoglobin’s heme pocket. Finally, we discuss our results and compare with previous calculations in the literature. Transition metal complexes play a fundamental role in many technological and biological areas. They act as catalysts in the oil industry during the processes of cracking and reforming, they are also fundamental for oxygen transport in blood, and they are key to plant respiration. Some of these processes can be well understood without the need to include the small contribution of spin-orbit (SO) interaction which, for the most part, is of the order of the error one performs in ab initio calculations of these kind of systems. In some cases, however, nonradiative transitions occur between states with different spins. Of particular interest are heme proteins, a biologically important group of molecules that have a unique common active site: an iron-protoporphyrin-IX complex, where changes in spin state can be an integral part of the protein function. One of the most striking biological manifestations of this modulation is the binding of molecular oxygen and carbon monoxide (CO) 1-9 to the five-coordinated ferrous heme site of globins. Another wellstudied example is the situation of several spin changes in the enzymatic activation cycles of cytochromes P450. These transitions are solely due to SO coupling and are otherwise forbidden. Ab initio calculations are performed within the framework of Hund’s Case A and hence assume that S, the total spin of the system, is a good quantum number. Spin-orbit coupling matrix elements could then be added, and rediagonalization of the Hamiltonian matrix would provide us with the spin coupled eigenstates of the system. Obtaining several electronic excited states for a given spin multiplicity is complex, and for mediumsized systems only configuration interaction for single excitations (CIS) is affordable. However, a more complete description of excited states might be required for the study of dynamics. For large systems, where the environment can be introduced by mixing quantum chemistry with molecular mechanics, electronically excited states are rarely computationally accessible and only the fundamental ground state for each spin multiplicity might be computed. For nonsymmetric systems, even at the density functional theory (DFT) level of theory, several attempts with different initial guesses should be considered in order to achieve the true ground state. The computational effort is drastically increased when several nuclear configurations are required for a complete study of the dynamics, and calculations on-the-fly, including excited states and SO coupling, are simply out of the question. Therefore, a semiempirical approach that includes excited states and SO coupling is very desirable. The method we develop in this paper is a generalization of Gerloch’s 10 cellular AOM (angular overlap model) version of ligand field theory, combined with a semiempirical version of Ellison’s 11-13 DIM (diatomics in molecules). Neither ligand field theory nor the semiempirical version of DIM are exact ab initio methods; not all the parameters can be obtained by analytical methods and they ought to be fitted either to experimental data or other calculations. On the other hand, once these parameters are known, fast computation of ground- and excited-state potentials can be performed without having to recalculate wave functions and their matrix elements. We test this method against a simplified model of the heme

Li + Li2 Dissociation Reaction Using the Self-Consistent Potential and Trajectory Surface Hopping Methods

Self-consistent potential and trajectory surface hopping methods have been applied to study the Li + Li2 dissociation reaction. Both methods fall into the classical trajectory methodology, with batches of 5000 trajectories being run over the translational energy range 25 ≤ Etr ≤ 100 kcal mol-1 keeping the internal state of Li2 fixed at (v = 0, j = 10). The effect of vibrational excitation has also been studied by running additional sets of trajectories for Etr = 25 kcal mol-1 with (v = 10, j = 10) and (v = 20, j = 10). All dissociative cross sections have been calculated using realistic double many-body expansion potential energy surfaces. The importance of nonadiabatic effects is investigated.

Practical formulation of accurate many-body potentials through the perturbative extension of diatomics-in-ionic-systems: Applied to HF clusters

A perturbative extension of the diatomics-in-ionic-systems (DIIS) is formulated as a practical method for describing global many-body potential energy surfaces with accuracy and economy. The method is applied to HF clusters, generalizing the prior accurate DIIS treatment of the dimer [Grigorenko, Nemukhin, and Apkarian, J. Chem. Phys. 108, 4413 (1998)] to arbitrary numbers of HF molecules. The calculated geometries, energetics, and harmonic frequencies of (HF)n, n=2–6 clusters agree in detail with the available data on this well-studied system. The formulation is based on treating intermolecular interactions within perturbation theory. It is shown that second-order perturbation, which includes bimolecular excitations, is necessary and sufficient in describing the many-body potential energy surfaces with spectroscopic accuracy. The approach allows the analysis of H-bonding and its nonadditive induction and dispersion forces in terms of mixings and exchange between ground- and excited states of dimers including intra- and intermolecular charge-transfer states as well as molecular triplet states. The speed of evaluation of the potential scales is the cube of the number of molecules, providing a practical method for dynamical simulations of extended hydrogen-bonded networks.

Potential Energy Surfaces for Chemical Reactions at Solid Surfaces

Many-body potential energy surfaces (PESs) for describing atomic interactions in gas-solid and surface reaction dynamics are reviewed in this work. Initial PESs from the 1960s-1970s were restricted to a diatomic molecule interacting with a solid surface. Since the 1980s, a multitude of many-body reactive PESs, their parameterization, and their applications have been reported in the literature. Although we mention most of the PESs in general, we have chosen to describe only those that either have had general utility or have had staying power, i.e. they have been used widely by other research groups. The potentials discussed in the most detail are the Stillinger-Weber and Tersoff Si PESs, the Brenner hydrocarbon PES, and the embedded-atom method (EAM) style potentials for metals. We conclude that although these PESs have been used successfully in large-scale computer simulations, further development is needed in many-body PESs. In particular, the development of new functional forms for multicomponent reactive systems is required.

Energetics of proton transfer in liquid water. I. Ab initio study for origin of many-body interaction and potential energy surfaces

The energetics of proton transfer in liquid water investigated by using ab initio calculation. The molecular electronic interaction of hydrated proton clusters in classified into many-body interaction elements by a new energy decomposition method. It is found that up to three-body molecular interaction is essential to describe the potential energy surface. The three-body effect mainly arises from the (non-classical) charge transfer and strongly depends on their configuration. Higher than three-body effects are small enough to be neglected. To simulate the liquid state reactions, two cluster models including all water molecules up to the second shell in the proton transfer reactions are employed. It is shown that these proton transfer reactions only involve small potential energy barriers of a few kcal/mol or less when structural rearrangement of the solvent is induced along the proton movement.

Quasiclassical trajectory calculations of the thermal rate coefficient for the oxygen atom + hydroxyl .fwdarw. oxygen + hydrogen atom reaction on realistic double many-body expansion potential energy surfaces for ground-state hydroperoxy

Quasi-classical trajectory calculations of the thermal rate coefficient for the title reaction have been carried out over the temperature range 250 less than or equal to T less than or equal to 2500 K by using two recently reported DMBE potential energy surfaces for the ground state of the hydroperoxyl radical. The results are compared with each other and with experiment. The agreement is good. The authors results support previous theoretical calculations by Miller on the Melius-Blint potential energy surface in that nonstatistical recrossing effects are very important. For the DMBE II (DMBE III) potential energy surface, these nonstatistical corrections are found to increase from a factor of about 1.2 (1.4) at 250 K about 2.1 (2.5) at 2500 K. However, they are considerably smaller than the nonstatistical corrections reported by Miller (factors of about 2.2 and 3.3 at the above temperatures). Although due, of course, to topographical differences between the DMBE and Melius-Blint potential energy surfaces, such discrepancy stems also from the different definitions used for HO/sub 2/* complex in the simple chemical model O + OH /r reversible/ HO/sub 2/* ..-->.. O/sub 2/ + H.