Showing papers on "Potential energy surface published in 2003"
TL;DR: The experiment directly probes the shared proton region of the potential energy surface and reveals three strong bands below 1600cm−1 and one at 1740 cm−1 (for H5O2 +), which highlight the importance of intermode coupling in sharedProton systems.
Abstract: The protonated water dimer is a prototypical system for the study of proton transfer in aqueous solution. We report infrared photodissociation spectra of cooled H + (H 2 O) 2 [and D + (D 2 O 2 ] ions, measured between 620 and 1900 wave numbers (cm −1 ). The experiment directly probes the shared proton region of the potential energy surface and reveals three strong bands below 1600 cm −1 and one at 1740 cm −1 (for H 5 O 2 + ). From a comparison to multidimensional quantum calculations, the three lower energy bands were assigned to stretching and bending fundamentals involving the O⋯H + ⋯O moiety, and the highest energy band was assigned to a terminal water bend. These results highlight the importance of intermode coupling in shared proton systems.
375 citations
TL;DR: Methods for geometry optimization of equilibrium structures, searching for transition states, following reaction paths and ab initio molecular dynamics are discussed, including methods for large molecules, QM/MM calculations, and simultaneous optimization of the wave function and the geometry.
Abstract: Potential energy surfaces form a central concept in the application of electronic structure methods to the study of molecular structures, properties, and reactivities. Recent advances in tools for exploring potential energy surfaces are surveyed. Methods for geometry optimization of equilibrium structures, searching for transition states, following reaction paths and ab initio molecular dynamics are discussed. For geometry optimization, topics include methods for large molecules, QM/MM calculations, and simultaneous optimization of the wave function and the geometry. Path optimization methods and dynamics based techniques for transition state searching and reaction path following are outlined. Developments in the calculation of ab initio classical trajectories in the Born-Oppenheimer and Car-Parrinello approaches are described.
318 citations
TL;DR: Using various forms of electronic-structure theory to characterize the important features of the potential energy surface, RRKM theory to calculate microcanonical rate coeffients, and several formulations of the master equation to predict phenomenological rate coefficients, this paper studied a number of reactions that occur on the C3H4 potential.
Abstract: Using various forms of electronic-structure theory to characterize the important features of the potential energy surface, RRKM theory to calculate microcanonical rate coeffients, and several formulations of the master equation to predict phenomenological rate coefficients, we have studied a number of reactions that occur on the C3H4 potential We discuss the results in some detail and compare them with experiment when possible Generally, the agreement with experiment is excellent “Multiple-well effects” are emphasized throughout the discussion We cast our results in the form of modified Arrhenius functions for use in chemical kinetics modeling
207 citations
TL;DR: The results of both analyses support the conclusion derived from previous mutational studies that the M20 loop of DHFR makes an important contribution to the electrostatic stabilization of the hydride transfer transition state.
Abstract: We have studied the hydride transfer reaction catalyzed by the enzyme dihydrofolate reductase (DHFR) and the coenzyme nicotinamide adenine dinucleotide phosphate (NADPH); the substrate is 5-protonated 7,8-dihydrofolate, and the product is tetrahydrofolate. The potential energy surface is modeled by a combined quantum mechanical-molecular mechanical (QM/MM) method employing Austin model 1 (AM1) and a simple valence bond potential for 69 QM atoms and employing the CHARMM22 and TIP3P molecular mechanics force fields for the other 21 399 atoms; the QM and MM regions are joined by two boundary atoms treated by the generalized hybrid orbital (GHO) method. All simulations are carried out using periodic boundary conditions at neutral pH and 298 K. In stage 1, a reaction coordinate is defined as the difference between the breaking and forming bond distances to the hydride ion, and a quasithermodynamic free energy profile is calculated along this reaction coordinate. This calculation includes quantization effects o...
199 citations
TL;DR: Bussery-Honvault et al. as mentioned in this paper used a Gaussian-weighted binning procedure to assign product quantum states in the QCT calculations, which yields vibrational branching ratios and rotational distributions in better agreement with the QM calculations than those obtained when the usual histogramatic binning method is employed.
Abstract: First accurate quantum mechanical (QM) calculations of integral and differential cross sections for the C(1D)+H2(v=0,j=0,1) insertion reaction have been performed on a newly developed ab initio potential energy surface [B. Bussery-Honvault et al., J. Chem. Phys. 115, 10701 (2001)]. These results have been compared with those obtained with a quasi-classical trajectory (QCT) method. A Gaussian-weighted binning procedure to assign product quantum states in the QCT calculations yields vibrational branching ratios and rotational distributions in better agreement with the QM calculations than those obtained when the usual histogramatic binning method is employed. This is the first time that the Gaussian-weighted binning procedure is used for an insertion reaction.
192 citations
TL;DR: In this article, the polarization-dependent differential cross sections for the Cl+H2 reaction were calculated on the BW2 potential energy surface, and the results indicated that the quasiclassical approximation in general does as good as exact quantum mechanics.
Abstract: Studies on the dynamical stereochemistry of the Cl+H2 reaction and its isotopic variants, especially the isotope effect on the product polarization, have been performed at a collision energy of 6.0 kcal/mol on two potential energy surfaces, i.e., G3 surface [T. C. Allison et al., J. Phys. Chem. 100, 13575 (1996)] and BW2 surface [W. Bian and H.-J. Werner, J. Chem. Phys. 112, 220 (2000)]. Quantum mechanical and quasiclassical trajectories calculations of the polarization-dependent differential cross sections for the Cl+H2 reaction have been carried out on the BW2 potential energy surface, and the results indicate that the quasiclassical approximation in general does as good as exact quantum mechanics. Calculations also show that the rotational alignment of the HCl product obtained on the BW2 surface for Cl+H2 reaction is stronger than that calculated on the G3 surface, which implies that the effect of van der Waals force on product polarization is quite weak. The distributions of P(θr) and P(φr) derived from the Cl+H2 and its isotopic reactions indicate that the isotope effect on the product polarization calculated on the G3 potential energy surface is distinct, whereas the isotope effect on the product polarization computed on the BW2 surface is indistinct.
178 citations
TL;DR: This is the first computation from first principles of a rate coefficient for a spin-forbidden reaction of a transition metal compound, and semi-quantitative agreement with experiment is obtained.
Abstract: New density functional theory and ab initio computations on the [Fe(CO)5] system are reported. Careful exploration of basis set and correlation effects leads to “best” values for the difference in energy ΔE(1,3) between ground state 3[Fe(CO)4] and the singlet excited state of ca. 8 kcal mol−1, and for the bond dissociation energy BDE(3) of [Fe(CO)5] with respect to ground state fragments 3[Fe(CO)4] + CO of ca. 40 kcal mol−1. A modified form of the B3PW91 functional is used to explore the potential energy surface for the spin-forbidden recombination reaction of CO with 3[Fe(CO)4]. A Cs-symmetric minimum energy crossing point (MECP) between the reactant (triplet) and product (singlet) potential energy surfaces is found, lying 0.43 kcal mol−1 above the reactants. The rate coefficient for recombination is computed using a non-adiabatic form of transition state theory, in which the MECP is treated as the critical point in the reaction. Semi-quantitative agreement with experiment is obtained: the predicted rate coefficient, 8.8 × 10−15 cm3 molecule−1 s−1, is only six times smaller than the experimental rate. This is the first computation from first principles of a rate coefficient for a spin-forbidden reaction of a transition metal compound.
148 citations
TL;DR: Experimental and theory now agree perfectly, within experimental error, bringing this 75-year-old scientific problem to completion, H+H2-->H2+H.
Abstract: New experimental and theoretical rate constants for two isotopologs of the simplest chemical reaction, H+H2-->H2+H, are presented. The theoretical results are obtained using accurate quantum dynamics with a converged Born-Oppenheimer potential energy surface and include non-Born-Oppenheimer corrections. The new experiments are carried out using a shock tube and complement earlier investigations over a very large T range, 167 to 2112 K. Experiment and theory now agree perfectly, within experimental error, bringing this 75-year-old scientific problem to completion.
129 citations
Book•
01 Jan 2003
TL;DR: In this article, the authors present a model of 3D surface reconstruction based on a 3D Brillouin Zones and a 2D-3D reconstruction of the surface.
Abstract: 1. Symmetry.- 1.1 Model Surfaces.- 1.1.1 Surface Versus Bulk.- 1.1.2 The Surface as a Physical Object.- 1.2 Two-Dimensional Crystals.- 1.2.1 Lattice Planes of Bulk Crystals.- 1.2.2 Oriented Slabs.- 1.2.3 Ideal Surfaces. Planar Point Groups.- 1.2.4 Real Surfaces: Reconstruction and Relaxation.- 1.2.5 Superlattices at Surfaces.- 1.2.6 Wood Notation.- 1.2.7 Symmetry Classification.- 1.3 Reciprocal Space.- 1.3.1 Direct and Reciprocal Lattices.- 1.3.2 Brillouin Zones.- 1.3.3 Projection of 3D Onto 2D Brillouin Zones.- 1.3.4 Symmetry of Points and Lines in Reciprocal Space.- 2. Thermodynamics.- 2.1 Kinetic Processes and Surfaces in Equilibrium.- 2.2 Thermodynamic Relations for Surfaces.- 2.2.1 Thermodynamic Potentials.- 2.2.2 Surface Modification of Thermodynamic Potentials.- 2.2.3 Surface Tension and Surface Stress.- 2.3 Equilibrium Shape of Small Crystals.- 2.3.1 Anisotropy of Surface Energy.- 2.3.2 Absolute Values for Surface Energies.- 2.3.3 Wulff Construction.- 2.4 Surface Energy and Morphology.- 2.4.1 Facetting and Roughening.- 2.4.2 3D Versus 2D Growth.- 2.4.3 Formation of Quantum Dots.- 2.5 Stoichiometry Dependence.- 2.5.1 Thermodynamic Approach.- 2.5.2 Approximations for Surface Energies.- 2.5.3 Chemical Potentials.- 2.5.4 Phase Diagrams.- 2.5.5 Stability of Adsorbates.- 3. Bonding and Energetics.- 3.1 Orbitals and Bonding.- 3.1.1 One-Electron Picture.- 3.1.2 Tight-Binding Approach.- 3.1.3 Atomic Orbitals and Their Interaction.- 3.1.4 Bonding Hybrids.- 3.1.5 Bonds and Bands.- 3.2 Dangling Bonds.- 3.2.1 Formation of Dangling Hybrids.- 3.2.2 Influence on Electronic States.- 3.3 Total Energy and Atomic Forces.- 3.3.1 Basic Approximations.- 3.3.2 Potential Energy Surface and Forces.- 3.3.3 Surface Diffusion.- 3.4 Quantitative Description of Structure and Stability.- 3.4.1 Density Functional Theory.- 3.4.2 Band-Structure and Interaction Contributions.- 3.4.3 Modeling of Surfaces.- 3.5 Bond Breaking: Accompanying Charge Transfers and Atomic Displacements.- 3.5.1 Characteristic Changes in Total Energy.- 3.5.2 Energy Gain Due to Structural and Configurational Changes.- 3.5.3 Energy Gain and Electron Transfer.- 4. Reconstruction Elements.- 4.1 Reconstruction and Bonding.- 4.1.1 Metallic Bonds.- 4.1.2 Strong Ionic Bonds.- 4.1.3 Mixed Covalent and Ionic Bonds.- 4.1.4 Principles of Semiconductor Surface Reconstruction.- 4.1.5 Electron Counting Rules.- 4.2 Chains.- 4.2.1 Zig-Zag Chains of Cations and Anions.- 4.2.2 ?-bonded Chains.- 4.2.3 Seiwatz Chains.- 4.3 Dimers.- 4.3.1 Symmetric Dimers.- 4.3.2 Asymmetric Dimers.- 4.3.3 Heterodimers.- 4.3.4 Bridging Groups.- 4.4 Adatoms and Adclusters.- 4.4.1 Isolated Adatoms.- 4.4.2 Adatoms Accompanied by Rest Atoms.- 4.4.3 Adatoms Combined with Other Reconstruction Elements.- 4.4.4 Trimers.- 4.4.5 Tetramers.- 5. Elementary Excitations I: Single Electronic Quasiparticles.- 5.1 Electrons and Holes.- 5.1.1 Excitation and Quasiparticle Character.- 5.1.2 Scanning Tunneling Spectroscopy.- 5.1.3 Photoemission Spectroscopy and Inverse Photoemmission.- 5.1.4 Satellites.- 5.2 Many-Body Effects.- 5.2.1 Quasiparticle Equation.- 5.2.2 Quasiparticle Shifts and Spectral Weights.- 5.2.3 Screening Near Surfaces.- 5.3 Quasiparticle Surface States.- 5.3.1 Surface Barrier.- 5.3.2 Characteristic Energies.- 5.3.3 State Localizaton.- 5.3.4 Quasiparticle Bands and Gaps.- 5.4 Strong Electron Correlation.- 5.4.1 Image States.- 5.4.2 Mott-Hubbard Bands.- 6. Elementary Excitations II: Pair and Collective Excitations.- 6.1 Probing Surfaces by Excitations.- 6.1.1 Optical Spectroscopies.- 6.1.2 Light Propagation in Surfaces.- 6.1.3 Electron Energy Losses.- 6.1.4 Raman Scattering.- 6.2 Electron-Hole Pairs: Excitons.- 6.2.1 Polarization function.- 6.2.2 Two-Particle Hamiltonian.- 6.2.3 Excitons.- 6.2.4 Surface Exciton Bound States.- 6.2.5 Surface-Modified Bulk Excitons.- 6.3 Plasmons.- 6.3.1 Intraband Excitations.- 6.3.2 Plasma Oscillations.- 6.3.3 Surface and Bulk Modes.- 6.4 Phonons.- 6.4.1 Harmonic Lattice Dynamics.- 6.4.2 Surface and Bulk Modes.- 6.4.3 Rayleigh Waves.- 6.4.4 Fuchs-Kliewer Phonons.- 6.4.5 Influence of Relaxation and Reconstruction.- 6.5 Elementary Excitations for Reduced Dimension.- 7. Defects.- 7.1 Realistic and Ideal Surfaces.- 7.2 Point Defects.- 7.2.1 Vacancies.- 7.2.2 Impurities.- 7.2.3 Antisites.- 7.3 Line Defects: Steps.- 7.3.1 Geometry and Notation.- 7.3.2 Steps on Si(100) Surfaces.- 7.3.3 Steps on Si(111) Surfaces.- 7.4 Planar Defects: Stacking Faults.- 7.4.1 Defect, Reconstruction Element or Bulk Property?.- 7.4.2 Si on Si(111)?3?3-B.- References.
128 citations
TL;DR: In this paper, a new potential energy surface for ozone was developed based on high level ab initio data and includes an accurate description of the barrier region, which is used for full quantum reactive scattering calculations using a coupled channel approach.
Abstract: A new potential energy surface for ozone is developed. It is based on high level ab initio data and includes an accurate description of the barrier region. Full quantum reactive scattering calculations using a coupled channel approach and hyperspherical coordinates are performed on this surface for various isotopic compositions of ozone. Collision lifetimes are obtained over a wide energy range, which gives the spectrum of rovibrational metastable states (scattering resonances). This spectrum is discovered to be very nonstatistical. The spectrum of resonances is dense below the isotopic zero-point-energy threshold and sparse above it. This feature is explained by the opening of additional dissociation channels at higher energies. This behavior is a general quantum mechanical effect that should occur in other triatomic molecules.
125 citations
TL;DR: The results demonstrate that for this system a purely classical description of the solvent is sufficient, since inclusion of the first solvent shell of 12 water molecules into the quantum system does not show a significant effect on this transition.
Abstract: The authors present a hybrid Car-Parrinello quantum mech./mol. mech. (QM/MM) approach that is capable of treating the dynamics of mol. systems in electronically excited states in complex environments. The potential energy surface in the excited state is described either within the restricted open-shell Kohn-Sham (ROKS) formalism or within time-dependent d. functional theory (TDDFT). As a test case, the authors apply this technique to the study of the solvent effects on the ground state and on the first excited singlet state of acetone in water. Results demonstrate that for this system a purely classical description of the solvent is sufficient, since inclusion of the first solvent shell of 12 water mols. into the quantum system does not show a significant effect on this transition. The excited-state energies calcd. with ROKS are red shifted by a const. value compared to the TDDFT results, while the relative variations of the excitation energy for different configurations are in very good agreement. The exptl. obsd. blue shift of the excitation energy in going from gas phase to condensed phase is well reproduced. Excited-state dynamics carried out with ROKS yield the relaxation of the solute and the rearrangement of the solvent structure on a picosecond timescale. The calcd. Stokes shift is in reasonable agreement with exptl. data. [on SciFinder (R)]
TL;DR: In this paper, the ground and excited-state properties of [Ru(bpy)(tpy)dmso] 2 + have been studied by the means of density functional theory (DFT), in particular, the singlet ground state and the potential energy surface of the lowest triplet were investigated along the coordinate involved in the S → O linkage isomerization of dmso.
Abstract: The ground and excited-state properties of [Ru(bpy)(tpy)dmso] 2 + (bpy = 2,2'-bipyridine, tpy = 2,2':6',2"-terpyridine; dmso = dimethyl sulfoxide) have been studied by the means of density functional theory (DFT). In particular, the singlet ground state and the potential energy surface of the lowest triplet were investigated along the coordinate involved in the S → O linkage isomerization of dmso. The time-dependent-DFT approach (TDDFT) was used to interpret the absorption spectra of the system, while a ΔSCF procedure was applied to compute the emission spectra. The good agreement between computed and experimental spectra highlights the power of DFT approaches in the description of complex transition metal containing systems. In addition, this method allows the full description of the ground and excited potential energy surfaces of [Ru(bpy)(tpy)-dmso] 2 + which can only be roughly derived from experimental data, thus providing clues for further improvement in the engineering of phototriggering materials.
TL;DR: General-gradient approximation and hybrid Hartree-Fock density functional theories in conjunction with basis sets of up to polarized triple-zeta quality have been applied to study the Stone-Wales transformation of buckminsterfullerene to yield a C(60) isomer of C(2)(v) symmetry with two adjacent pentagons.
Abstract: General-gradient approximation (PBE) and hybrid Hartree−Fock density functional theories (B3LYP) in conjunction with basis sets of up to polarized triple-ζ quality have been applied to study the Stone−Wales transformation of buckminsterfullerene (BF) to yield a C60 isomer of C2v symmetry with two adjacent pentagons (#1809). In agreement with earlier investigations, two different transition states and reaction pathways could be identified for the rearrangement from BF to C60-C2v on the C60 potential energy surface (PES). One has C2 molecular point group symmetry with the two migrating carbon atoms remaining close to the fullerene surface. The other one has a high-energy carbene-like (sp3) structure where a single carbon atom is significantly moved away from the C60 surface. The carbene intermediate and the second transition state along the stepwise reaction path characterized previously at lower levels of theory do not exist as stationary points with the density functionals utilized here. The classical bar...
TL;DR: In this article, the authors show that the DFT/GGA method is not qualitatively accurate for predicting the most stable CO adsorption sites on metal surfaces, and that for energy differences smaller than say 0.1 eV, calculated quantities like vibrational frequencies and geometries discriminate correctly between sites, being in agreement with experiments at the correct adorption site.
Abstract: Today’s state-of-the-art method for calculating the interaction of atoms or small molecules with metal surfaces is considered to be density functional theory (DFT) at the generalized gradient approximation (GGA) level employing a slab or supercell representation of the surface. The method is widely used and by many assumed to be both qualitatively and quantitatively accurate. This notion has recently been challenged by Feibelman et al. [J. Phys. Chem. B 105, 4018 (2001)] who suggest that the DFT/GGA method does not correctly predict the most stable adsorption site for the CO/Pt(111) system, and they conclude that the method is not qualitatively accurate. However, using a different calculational approach we find a good agreement between the calculated potential energy surface for this system and the one inferred from experiments, indicating that the evidence supporting the view of Feibelman et al. is not yet conclusive. On the contrary, we advocate the view that the DFT/GGA method should at the moment be considered qualitatively accurate for predicting the most stable CO adsorption sites on metal surfaces. This view is supported by (i) our results for the Pt(111) surface which in agreement with experiments favors the top site, (ii) an assessment of literature results for other surfaces, suggesting that the error in the relative stability of the CO adsorption sites on a given surface is within ±0.1 eV when compared to experiments, (iii) the considerable challenge one faces when trying to converge DFT/GGA calculations within ±0.1 eV with respect to all computational parameters, (iv) and that for energy differences smaller than say 0.1 eV, calculated quantities like, e.g., vibrational frequencies and geometries discriminate correctly between sites, being in agreement with experiments at the correct adsorption site.
TL;DR: In this article, the relative energies of stationary points on the potential energy surface of trans-stilbene have been accurately determined using Hartree-Fock, second and third-order Moller-Plesset (MP2, MP3), as well as Coupled Clusters theories with single and double excitations (CCSD), together with a perturbative estimate of connected triple excitations [CCSD(T), in conjunction with basis sets of increasing size, containing up to 1130 basis functions.
Abstract: The relative energies of stationary points on the potential energy surface of trans-stilbene have been accurately determined using Hartree–Fock, second and third-order Moller–Plesset (MP2, MP3), as well as Coupled Clusters theories with single and double excitations (CCSD), together with a perturbative estimate of connected triple excitations [CCSD(T)], in conjunction with basis sets of increasing size, containing up to 1130 basis functions. A focal point analysis has been carried out in order to determine how the energy differences and rotational barriers approach convergence, enabling extrapolation of the CCSD(T) results to a near-complete basis set. The investigated saddle points pertain to independent rotations of the phenyl rings about the single C–C bond, and to pedalling motions described by a twofold rotation of the central ethylene bond about the longitudinal axis of the molecule. The benchmark calculations presented in this study lead to the conclusion that, in the nonrelativistic limit and within the frozen core approximation, trans-stilbene in vacuum is a strictly planar molecule in its absolute energy minimum form, in sharp contrast with many previous theoretical studies. This point has been ultimately confirmed by an MP2 geometry optimization using the aug-cc-pVDZ basis set. At last, the energy of cis-stilbene relative to the trans-isomer is accurately evaluated.
TL;DR: In this paper, a search for low energy structures of water clusters was performed with a combination of three computational tools: (a) temperature-dependent classical trajectories; (b) hydrogen network improvement; (c) rigid body diffusion Monte Carlo calculation on a smoothed potential energy surface.
Abstract: A search for low energy structures of water clusters was performed with a combination of three computational tools: (a) temperature-dependent classical trajectories; (b) hydrogen network improvement; (c) rigid body diffusion Monte Carlo calculation on a smoothed potential energy surface. For the sizes of our main interest, n = 48, 123, and 293, input configurations included spheroid structures cut from crystalline ice, and amorphous structures. For n = 48, tube and sandwich minima were explored as well. The lowest energy configurations found were characterized by compact three-dimensional shapes. In the case of n = 48 and 123, crystallinity was lost in the course of the optimization; for these sizes, one finds four-, five-, and six-membered rings of water molecules, On the other hand, the lowest energy structure found for n = 293 includes a crystal core, dominated by six-membered rings, and an amorphous surface.
TL;DR: In this article, the authors apply multireference ab initio quantum chemistry and microcanonical transition state theory with quantum energy flow corrections from local random matrix theory (LRMT) to determine the kinetics of trans-stilbene photoisomerization.
Abstract: We apply multireference ab initio quantum chemistry and microcanonical transition state (RRKM) theory with quantum energy flow corrections from local random matrix theory (LRMT) to determine the kinetics of trans-stilbene photoisomerization. With a single ab initio potential energy surface and no adjustable parameters, simultaneous agreement with experiment of the microcanonical isomerization rates for the d0, d2, d10, and d12 isotopomers is obtained. We are also able to reproduce the pressure dependence of the thermal rate. Laser cooling effects on the isomerization rate are calculated and found to be quite small. The S1/S2 energy gap at the transition state is found to be quite large (0.86 eV), suggesting that nonadiabatic effects are negligible. Using the ab initio results in a simple RRKM theory without corrections for finite quantum energy flow does not lead to agreement with experiment. We conclude that non-RRKM effects are essential to understand photoisomerization of trans-stilbene and that these ...
TL;DR: In this article, a potential energy surface for the major isotopomer of water is constructed by fitting to observed vibration-rotation energy levels of the system using the exact kinetic energy operator nuclear motion program DVR3D.
Abstract: A potential energy surface for the major isotopomer of water is constructed by fitting to observed vibration–rotation energy levels of the system using the exact kinetic energy operator nuclear motion program DVR3D. The starting point for the fit is the ab initio Born–Oppenheimer surface of Partridge and Schwenke [J. Chem. Phys. 106, 4618 (1997)] and corrections to it: both one- and two-electron relativistic effects, a correction to the height of the barrier to linearity, allowance for the Lamb shift and the inclusion of both adiabatic and nonadiabatic non-Born–Oppenheimer corrections. Fits are made by scaling the starting potential by a morphing function, the parameters of which are optimized. Two fitted potentials are presented which only differ significantly in their treatment of rotational nonadiabatic effects. Energy levels up to 25 468 cm−1 with J=0, 2, and 5 are fitted with only 20 parameters. The resulting potentials predict experimentally known levels with J⩽10 with a standard deviation of 0.1 cm−1, and are only slightly worse for J=20, for which rotational nonadiabatic effects are significant. The fits showed that around 100 known energy levels are probably the result of misassignments. Analysis of misassigned levels above 20 000 cm−1 leads to the reassignment of 23 transitions.
TL;DR: Using a wave packet method, state-to-state inelastic transition probabilities and initial state specified total reaction probabilities are calculated for the title system (J=0) on a recent ab initio potential energy surface as mentioned in this paper.
Abstract: Using a wave packet method, state-to-state inelastic transition probabilities and initial state specified total reaction probabilities are calculated for the title system (J=0) on a recent ab initio potential energy surface. Both the inelastic and reactive scattering probabilities are found to be strongly oscillatory, indicative of the involvement of long-lived resonances that are supported by a deep CH2 well. The oscillation becomes less pronounced at higher collision energies and with internal excitation of the reactant molecule. The reaction from the (νi=0, ji=0) initial state is clearly dominated by the insertion pathway, and this dominance is largely unaffected by the excitation of the reactant rotation or vibration. In addition, low-lying vibrational states of CH2 have been determined and compared with spectroscopic data.
TL;DR: In this paper, a potential energy surface and calculations of power spectra for CH5+ were obtained by precise fitting of MP2/cc-pVTZ electronic energies and gradients, which were obtained in classical direct-dynamics calculations.
Abstract: We report a potential energy surface and calculations of power spectra for CH5+. The potential surface is obtained by precise fitting of MP2/cc-pVTZ electronic energies and gradients, which are obtained in classical direct-dynamics calculations. The power spectra are obtained using standard microcanonical classical and novel quasiclassical calculations of the velocity autocorrelation function, from which the power spectrum is obtained in the usual way. Both calculations agree qualitatively that the overall spectrum is quite complex; however, the latter calculations indicate that some spectral features may be assignable.
TL;DR: In this article, a new full-dimensional potential energy surface has been constructed to describe the OH+CO↔H+CO2 reactive system, which modifies the existing many body expansion potential of Bradley and Schatz based on recent ab initio calculations and incorporates an entirely new hybrid surface to accurately describe the entrance channel and two possible van der Waals complexes.
Abstract: A new, full-dimensional potential energy surface has been constructed to describe the OH+CO↔H+CO2 reactive system. The new surface modifies the existing many body expansion potential of Bradley and Schatz based on recent ab initio calculations and incorporates an entirely new hybrid surface to accurately describe the OH+CO entrance channel and two possible van der Waals complexes, OH–CO and OH–OC. Quasiclassical trajectory calculations have been performed for the reaction OH+CO→H+CO2 using the new surface in order to examine the impact of the changes in the surface, to evaluate the accuracy of the surface by comparing to experimental results, and to investigate the reaction dynamics of this interesting complex-forming system. It is shown that the improvement in the description of the entrance channel has a rather large effect on overall reactivity and response to reagent rotational and vibrational excitation, but has little effect on various product properties such as angular and translational energy dist...
TL;DR: Ojamae et al. as discussed by the authors investigated the vibrational mode coupling in the protonated water dimer by performing two types of quantum calculations of vibrational levels of H5O 2+ and D5O2+, utilizing the OSS3(p) potential energy surface.
Abstract: The vibrational mode coupling in the protonated water dimer is investigated by performing two types of quantum calculations of the vibrational levels of H5O2+ and D5O2+, utilizing the OSS3(p) potential energy surface by Ojamae et al. [L. Ojamae, I. Shavitt, and S. J. Singer, J. Chem. Phys. 109, 5547 (1998)]. One is four-dimensional (4D), treating only the central O⋯H(D)+⋯O moiety. Three of the four modes considered, the asymmetric stretch and the two bends, are largely the vibrations of the central proton, while the fourth mode is essentially the O⋯O stretching vibration. The vibrational levels of O⋯H(D)+⋯O are calculated rigorously, as fully coupled (FC), and also in an adiabatic (3+1)D approximation, where the proton asymmetric stretch is treated as adiabatically separated from the other three degrees of freedom. The second set of calculations, designated VCI, is full-dimensional, 15D; it is performed by the code MULTIMODE, which does configuration interaction (CI) calculations using a basis determined ...
TL;DR: A new form of kinetic isotope effect is demonstrated, unrelated to the usual effect of zero-point energies on barriers, in the ene reaction of singlet oxygen with tetramethylethylene using quasiclassical direct dynamics calculations on a B3LYP/6-31G* potential energy surface.
Abstract: The intramolecular H/D kinetic isotope effect in the ene reaction of singlet oxygen with tetramethylethylene is studied using quasiclassical direct dynamics calculations on a B3LYP/6-31G* potential energy surface. Starting from the area of the energy surface around a valley-ridge inflection point, random trajectories lead to predominantly H abstraction over D abstraction, despite the symmetry of the surface and the absence of a barrier to either reaction. This demonstrates a new form of kinetic isotope effect, unrelated to the usual effect of zero-point energies on barriers. Dynamics calculations on the reaction of cis-2-pentene predict the experimentally observed mixture of regioisomeric products, while the minimum-energy path leads to only one product. For energy surfaces containing two adjacent saddle points, dynamics effects are important for understanding both product and isotopic selectivity, and this should be considered in the interpretation of experimental results.
TL;DR: In this article, the isotope dependence of the O+O2 exchange reaction was investigated by means of kinetic experiments and classical trajectory calculations on an accurate potential energy surface, and the measurements confirmed the previously reported negative temperature dependence and yield the rate coefficients for both the exothermic 18O+16O2→18O16O +16O and the endothermic 16O+18O2 →16O18O +18O reaction between 233 and 353 K: k8=(3.4±0.5 cm3)
Abstract: The isotope dependence of the O+O2 exchange reaction is investigated by means of kinetic experiments and classical trajectory calculations on an accurate potential energy surface. The measurements confirm the previously reported negative temperature dependence and yield the rate coefficients for both the exothermic 18O+16O2→18O16O+16O and the endothermic 16O+18O2→16O18O+18O reaction between 233 and 353 K: k8=(3.4±0.6)×10−12 (300 K/T)1.1±0.5 cm3 s−1 and k6=(2.7±0.4)×10−12 (300 K/T)0.9±0.5 cm3 s−1. In addition, the ratio of these two rates, R, has been measured with comparatively higher precision. It is 1.27±0.04 at 300 K and also shows a distinct negative temperature dependence. Four types of classical trajectory calculations are performed in order to interpret the experimental result. They differ by the way in which the quantum mechanical zero-point energy of the reactants and the differences of zero-point energies between reactants and products, ΔEZPE≈±22 cm−1, are phenomenologically incorporated. Only c...
TL;DR: In this article, the authors presented a time-dependent quantum wave packet calculation for the reaction of F(2P3/2,2P1/2)+H2 on the Alexander-Stark-Werner potential energy surface.
Abstract: In this paper we present a time-dependent quantum wave packet calculation for the reaction of F(2P3/2,2P1/2)+H2 on the Alexander–Stark–Werner potential energy surface. The reaction probabilities and the integral cross sections for the reaction of F(2P3/2,2P1/2)+H2 (v=j=0) are computed using time-dependent quantum methods with the centrifugal sudden approximate. The results are compared with recent time-independent quantum calculations. The two-surface reaction probability for the initial ground spin–orbit state of J=0.5 is similar to the time-independent result obtained by Alexander et al. [J. Chem. Phys. 113, 11084 (2000)]. Our calculation also shows that electronic coupling has a relatively minor effect on the reactivity from the 2P3/2 state but a non-negligible one from the 2P1/2 state. By comparison with exact time-independent calculations, it is found that the Coriolis coupling plays a relatively minor role. In addition, most of the reactivity of the excited state of fluorine atom results from the sp...
TL;DR: In this paper, the authors discuss different definitions of the reaction path, their merits as well as their drawbacks: IRC (steepest descent from saddle), reduced gradient following (RGF), gradient extremals, and some others.
Abstract: The reaction path is an important concept in theoretical chemistry. We discuss different definitions, their merits as well as their drawbacks: IRC (steepest descent from saddle), reduced gradient following (RGF), gradient extremals, and some others. Many properties and problems are explained by two-dimensional figures. This paper is both a review and a pointer to future research. The branching points of RGF curves are valley-ridge inflection (VRI) points of the potential energy surface. These points may serve as indicators for bifurcations of the reaction path. The VRI points are calculated with the help of Branin's method. All the important features of the potential energy surface are independent of the coordinate system. Besides the theoretical definitions, we also discuss the numerical use of the methods.
TL;DR: In this article, a new potential energy surface for C 2 H 2 that describes acetylene/vinylidene isomerization is reported. But the surface is an accurate, least-squares fit to nearly 10,000 symmetry-equivalent, ab initio electronic calculations obtained at the CCSD(T) level of theory, with an aug-cc-pVTZ basis.
TL;DR: In this paper, the authors applied molecular dynamics umbrella-sampling simulations and ensemble-averaged variational transition state theory with multidimensional tunneling (EA-VTST/MT) to explore the free energy surface, reaction paths, and dideuterium kinetic isotope effect (KIE) of the β-oxidation of butyryl-coenzyme A by short-chain acyl-CoA dehydrogenase.
Abstract: We have applied molecular dynamics umbrella-sampling simulations and ensemble-averaged variational transition state theory with multidimensional tunneling (EA-VTST/MT) to explore the free energy surface, reaction paths, and dideuterium kinetic isotope effect (KIE) of the β-oxidation of butyryl-coenzyme A by short-chain acyl-CoA dehydrogenase. The potential energy surface is obtained by combined quantum mechanics−molecular mechanics (QM/MM) with specific reaction parameters and a simple valence bond term. The calculations include determination of the potential of mean force (PMF) in both two dimensions (2D) and one dimension (1D) by using the weighted histogram analysis method. The 2D PMF indicates that the hydride transfer is the rate-limiting step of the mechanism, and the 1D PMFs are used to calculate rate constants. We include full molecular dynamics of a 7824-atom reaction zone with stochastic boundary conditions in a first approximation of the quasiclassical rate constant. This first approximation is...
TL;DR: In this article, cross sections for rotational, vibrational, and fine-structure transitions in He-CaH(2Σ) collisions at cold and ultracold temperatures calculated using the ab initio potential energy surface reported in the preceding paper were presented.
Abstract: We present cross sections for rotational, vibrational, and fine-structure transitions in He–CaH(2Σ) collisions at cold and ultracold temperatures calculated using the ab initio potential energy surface reported in the preceding paper. Rotational quenching is fast, vibrational quenching is slow. The spin-rotational interaction, although small and having no influence at temperatures above 10 K, changes significantly the rate coefficients for rotational quenching at lower temperatures. The theoretical rotational, vibrational, and elastic cross sections are compared with the results of a buffer gas cooling experiment carried out at a temperature of about 0.4 K. The theoretical predictions for the vibrational and elastic cross sections are larger than the measured values. The sensitivity to the potential energy surface is explored. A modified surface diminishes but does not remove the differences between theory and experiment.
TL;DR: In this article, the N + N2 exchange rate is calculated using a time-dependent quantum dynamics method on a newly determined ab initio potential energy surface (PES) for the ground A" state.
Abstract: The N + N2 exchange rate is calculated using a time-dependent quantum dynamics method on a newly determined ab initio potential energy surface (PES) for the ground A" state. This ab initio PES shows a double barrier feature in the interaction region with the barrier height at 47.2 kcal/mol, and a shallow well between these two barriers, with the minimum at 43.7 kcal/mol. A quantum dynamics wave packet calculation has been carried out using the fitted PES to compute the cumulative reaction probability for the exchange reaction of N + N2(J=O). The J - K shift method is then employed to obtain the rate constant for this reaction. The calculated rate constant is compared with experimental data and a recent quasi-classical calculation using a LEPS PES. Significant differences are found between the present and quasiclassical results. The present rate calculation is the first accurate 3D quantal dynamics study for N + N2 reaction system and the ab initio PES reported here is the first such surface for N3.