Ab initio global potential-energy surface for H5(+) --> H3(+) + H2

An accurate global potential-energy surface (PES) is reported for H5(+) based on more than 100,000 CCSD(T)/aug-cc-pVTZ ab initio energies. This PES has full permutational symmetry with respect to interchange of H atoms and dissociates to H3(+) and H2. Ten known stationary points of H5(+) are characterized and compared to previous ab initio calculations. Quantum diffusion Monte Carlo calculations are performed on the PES to obtain the zero-point energy of H5(+) and the anharmonic dissociation energy (D0) of H5(+) --> H3(+) + H2. The rigorous zero-point state of H4D+ is also calculated and discussed within the context of a strictly classical approach to obtain the branching ratio of the reaction H4D+ --> H3(+) + HD and H2D+ + H2. Such an approach is taken using the PES and critiqued based on the properties of the quantum zero-point state. Finally, a simple procedure for adding the long range-interaction energy is described.     

Overtone-induced dissociation and isomerization dynamics of the hydroxymethyl radical (CH2OH and CD2OH). I. A theoretical study

The dissociation of the hydroxymethyl radical, CH(2)OH, and its isotopolog, CD(2)OH, following the excitation of high OH stretch overtones is studied by quasi-classical molecular dynamics calculations using a global potential energy surface (PES) fitted to ab initio calculations. The PES includes CH(2)OH and CH(3)O minima, dissociation products, and all relevant barriers. Its analysis shows that the transition states for OH bond fission and isomerization are both very close in energy to the excited vibrational levels reached in recent experiments and involve significant geometry changes relative to the CH(2)OH equilibrium structure. The energies of key stationary points are refined using high-level electronic structure calculations. Vibrational energies and wavefunctions are computed by coupled anharmonic vibrational calculations. They show that high OH-stretch overtones are mixed with other modes. Consequently, trajectory calculations carried out at energies about ~3000 cm(-1) above the barriers reveal that despite initial excitation of the OH stretch, the direct OH bond fission is relatively slow (10 ps) and a considerable fraction of the radicals undergoes isomerization to the methoxy radical. The computed dissociation energies are: D(0)(CH(2)OH → CH(2)O + H) = 10,188 cm(-1), D(0)(CD(2)OH → CD(2)O + H) = 10,167 cm(-1), D(0)(CD(2)OH → CHDO + D) = 10,787 cm(-1). All are in excellent agreement with the experimental results. For CH(2)OH, the barriers for the direct OH bond fission and isomerization are: 14,205 and 13,839 cm(-1), respectively.     

An ab initio based global potential energy surface describing CH5+ --> CH3+ + H2

A full-dimensional, ab initio based potential energy surface (PES) for CH(5)(+), which can describe dissociation is reported. The PES is a precise fit to 36173 coupled-cluster [CCSD(T)] calculations of electronic energies done using an aug-cc-pVTZ basis. The fit uses a polynomial basis that is invariant with respect to permutation of the five H atoms, and thus describes all 120 equivalent minima. The rms fitting error is 78.1 cm(-1) for the entire data set of energies up to 30,000 cm(-1) and a normal-mode analysis of CH(5)(+) also verifies the accuracy of the fit. Two saddle points have been located on the surface as well and compared with previous theoretical work. The PES dissociates correctly to the fragments CH(3)(+) + H(2) and the equilibrium geometry and normal-mode analyses of these fragments are also presented. Diffusion Monte Carlo calculations are done for the zero-point energies of CH(5)(+) (and some isotopologs) as well as for the separated fragments of CH(5)(+), CH(3)(+) + H(2) and those of CH(4)D(+), CH(3)(+) + HD and CH(2)D(+) + H(2). Values of D(0) are reported for these dissociations. A molecular dynamics calculation of CH(4)D(+) dissociation at one total energy is also performed to both validate the applicability of the PES for dynamics studies as well as to test a simple classical statistical prediction of the branching ratio of the dissociation products.     

Quasiclassical trajectory calculations of the OH+NO2 association reaction on a global potential energy surface

We report a full-dimensional potential energy surface (PES) for the OH+NO(2) reaction based on fitting more than 55,000 energies obtained with density functional theory-B3LYP6-311G(d,p) calculations. The PES is invariant with respect to permutation of like nuclei and describes all isomers of HOONO, HONO(2), and the fragments OH+NO(2) and HO(2)+NO. Detailed comparison of the structures, energies, and harmonic frequencies of various stationary points on the PES are made with previous and present high-level ab initio calculations. Two hydrogen-bond complexes are found on the PES and confirmed by new ab initio CASPT2 calculations. Quasiclassical trajectory calculations of the cross sections for ground rovibrational OH+NO(2) association reactions to form HOONO and HONO(2) are done using this PES. The cross section to form HOONO is larger than the one to form HONO(2) at low collision energies but the reverse is found at higher energies. The enhancement of the HOONO complex at low collision energies is shown to be due, in large part, to the transient formation of a H-bond complex, which decays preferentially to HOONO. The association cross sections are used to obtain rate constants for formation of HOONO and HONO(2) for the ground rovibrational states in the high-pressure limit.     

Quasiclassical trajectory calculations of the reaction C+C2H2-->l-C3H, c-C3H+H, C3+H2 using full-dimensional triplet and singlet potential energy surfaces

Full-dimensional, density functional theory (B3LYP/6-311g(d,p))-based potential energy surfaces (PESs) are reported and used in quasi-classical calculations of the reaction of C with C(2)H(2). For the triplet case, the PES spans the region of the reactants, the complex region (with numerous minima and saddle points) and the products, linear(l)-C(3)H+H, cyclic(c)-C(3)H+H and c-(3)C(3)+H(2). For the singlet case, the PES describes the complex region and products l-C(3)H+H, c-C(3)H+H and l-(1)C(3)+H(2). The PESs are invariant under permutation of like nuclei and are fit to tens of thousands of electronic energies. Energies and harmonic frequencies of the PESs agree well the DFT ones for all stationary points and for the reactant and the products. Dynamics calculations on the triplet PES find both l-C(3)H and c-C(3)H products, with l-C(3)H being dominant at the energies considered. Limited unimolecular reaction dynamics on the singlet PES find both products in comparable amounts as well as the C(3)+H(2) product.     

MULTIMODE quantum calculations of intramolecular vibrational energies of the water dimer and trimer using ab initio-based potential energy surfaces

We report vibrational configuration interaction calculations of the monomer fundamentals of (H(2)O)(2), (D(2)O)(2), (H(2)O)(3), and (D(2)O)(3) using the code MULTIMODE and full dimensional ab initio-based global potential energies surfaces (PESs). For the dimer the HBB PES [Huang et al., J. Chem. Phys 128, 034312 (2008)] is used and for the trimer a new PES, reported here, is used. The salient properties of the new trimer PES are presented and compared to previous single-point calculations and the vibrational energies are compared with experiments.     

Ab initio potential energy and dipole moment surfaces for H5O2 +

Full-dimensional ab initio potential energy surface (PES) and dipole moment surface (DMS) are reported for H(5)O(2) (+). Tens of thousands of coupled-cluster [CCSD(T)] and second-order Moller-Plesset (MP2) calculations of electronic energies, using aug-cc-pVTZ basis, were done. The energies were fit very precisely in terms of all the internuclear distances, using standard least-square procedures, however, with a fitting basis that satisfies permutational symmetry with respect to like atoms. The H(5)O(2) (+) PES is a fit to 48 189 CCSD(T) energies, containing 7962 polynomial coefficients. The PES has a rms fitting error of 34.9 cm(-1) for the entire data set up to 110 000 cm(-1). This surface can describe various internal floppy motions, including the H atom exchanges, monomer inversions, and monomer torsions. First- and higher-order saddle points have been located on the surface and compared with available previous theoretical work. In addition, the PES dissociates correctly (and symmetrically) to H(2)O+H(3)O(+), with D(e)=11 923.8 cm(-1). Geometrical and vibrational properties of the monomer fragments are presented. The corresponding global DMS fit (MP2 based) involves 3844 polynomial coefficients and also dissociates correctly.     

用含时波包方法将SVRT模型应用到F+CD4→CD3+DF反应中

In order to study the quantum reaction dynamics of large molecular systems, the time dependent quantum wave packet approach was used to study the F+CD4→CD3+DF reaction systems. The semirigid vibrating rotor model proposed by J.Z.H. Zhang was used on the MJ1 potential energy surface. The barrier height of the MJ1 PES was about 66 meV. In the semirigid vibrating rotor model, the fragment CD3 was fixed to in the geometry, its transition state value, because from the reactant to the transition state the C-D bond in the CD3 group almost remains constant, which can be treated as a spectator bond. The numerical calculation showed that there were oscillatory structures in the energy dependence of the calculated integral cross section. Those structures are generally associated with dynamic resonances. Cross section and rate constant were calculated based on the MJ1 PES of the ground state. These results are comparable to the results of previous calculations and reaction dynamic experiment results. At low temperature and collision energies, the tunneling effect works most remarkably in the reaction process to make the D abstraction easier. At high temperatures and collision energies, the rate constant is higher than the experimental results.     

Quasiclassical trajectory study of the F + CH4 reaction dynamics on a dual-level interpolated potential energy surface

An ab initio interpolated potential energy surface (PES) for the F + CH4 reactive system has been constructed using the interpolation method of Collins and co-workers. The ab initio calculations have been performed using second-order M?ller-Plesset (MP2) perturbation theory to build the initial PES. Scaling all correlation (SAC) methodology has been employed to improve the ab initio calculations and to construct a dual-level PES. Using this PES, a detailed quasiclassical trajectory study of integral and differential cross sections, product rovibrational populations and internal energy distributions has been carried out for the F + CH4 and F + CD4 reactions and the theoretical results have been compared with the available experimental data.     

High-dimensional ab initio potential energy surfaces for reaction dynamics calculations

There has been great progress in the development of potential energy surfaces (PESs) for reaction dynamics that are fits to ab initio energies. The fitting techniques described here explicitly represent the invariance of the PES with respect to all permutations of like atoms. A review of a subset of dynamics calculations using such PESs (currently 16 such PESs exist) is then given. Bimolecular reactions of current interest to the community, namely, H + CH(4) and F + CH(4), are focused on. Unimolecular reactions are then reviewed, with a focus on the photodissociation dynamics of H(2)CO and CH(3)CHO, where so-called "roaming" pathways have been discovered. The challenges for electronically non-adiabatic reactions, and associated PESs, are presented with a focus on the OH* + H(2) reaction. Finally, some thoughts on future directions and challenges are given.     

Accurate ab initio potential energy surface, thermochemistry, and dynamics of the Cl(2P, 2P(3/2) + CH4 → HCl + CH3 and H + CH3Cl reactions

We report a high-quality, ab initio, full-dimensional global potential energy surface (PES) for the Cl((2)P, (2)P(3/2)) + CH(4) reaction, which describes both the abstraction (HCl + CH(3)) and substitution (H + CH(3)Cl) channels. The analytical PES is a least-squares fit, using a basis of permutationally invariant polynomials, to roughly 16,000 ab initio energy points, obtained by an efficient composite method, including counterpoise and spin-orbit corrections for the entrance channel. This composite method is shown to provide accuracy almost equal to all-electron CCSD(T)/aug-cc-pCVQZ results, but at much lower computational cost. Details of the PES, as well as additional high-level benchmark characterization of structures and energetics are reported. The PES has classical barrier heights of 2650 and 15,060 cm(-1) (relative to Cl((2)P(3/2)) + CH(4)(eq)), respectively, for the abstraction and substitution reactions, in good agreement with the corresponding new computed benchmark values, 2670 and 14,720 cm(-1). The PES also accurately describes the potential wells in the entrance and exit channels for the abstraction reaction. Quasiclassical trajectory calculations using the PES show that (a) the inclusion of the spin-orbit corrections in the PES decreases the cross sections by a factor of 1.5-2.5 at low collision energies (E(coll)); (b) at E(coll) ≈ 13,000 cm(-1) the substitution channel opens and the H/HCl ratio increases rapidly with E(coll); (c) the maximum impact parameter (b(max)) for the abstraction reaction is ~6 bohr; whereas b(max) is only ~2 bohr for the substitution; (d) the HCl and CH(3) products are mainly in the vibrational ground state even at very high E(coll); and (e) the HCl rotational distributions are cold, in excellent agreement with experiment at E(coll) = 1280 cm(-1).     

Quasiclassical trajectory study of the Cl+CH4 reaction dynamics on a quadratic configuration interaction with single and double excitation interpolated potential energy surface

An ab initio interpolated potential energy surface (PES) for the Cl+CH(4) reactive system has been constructed using the interpolation method of Collins and co-workers [J. Chem. Phys. 102, 5647 (1995); 108, 8302 (1998); 111, 816 (1999); Theor. Chem. Acc. 108, 313 (2002)]. The ab initio calculations have been performed using quadratic configuration interaction with single and double excitation theory to build the PES. A simple scaling all correlation technique has been used to obtain a PES which yields a barrier height and reaction energy in good agreement with high level ab initio calculations and experimental measurements. Using these interpolated PESs, a detailed quasiclassical trajectory study of integral and differential cross sections, product rovibrational populations, and internal energy distributions has been carried out for the Cl+CH(4) and Cl+CD(4) reactions, and the theoretical results have been compared with the available experimental data. It has been shown that the calculated total reaction cross sections versus collision energy for the Cl+CH(4) and Cl+CD(4) reactions is very sensitive to the barrier height. Besides, due to the zero-point energy (ZPE) leakage of the CH(4) molecule to the reaction coordinate in the quasiclassical trajectory (QCT) calculations, the reaction threshold falls below the barrier height of the PES. The ZPE leakage leads to CH(3) and HCl coproducts with internal energy below its corresponding ZPEs. We have shown that a Gaussian binning (GB) analysis of the trajectories yields excitation functions in somehow better agreement with the experimental determinations. The HCl(v'=0) and DCl(v'=0) rotational distributions are as well very sensitive to the ZPE problem. The GB correction narrows and shifts the rotational distributions to lower values of the rotational quantum numbers. However, the present QCT rotational distributions are still hotter than the experimental distributions. In both reactions the angular distributions shift from backward peaked to sideways peaked as collision energy increases, as seen in the experiments and other theoretical calculations.     

Influence of the potential energy surface on the reaction cross section: the K + HF → KF + H system

The effect of the potential energy surface on the K + HF → KF + H cross section has been studied using reasonable Sorbie-Murrell (bent saddle point) and LEPS (collinear saddle point) potential energy surfaces (PESs). Trajectory calculations for selected initial conditions (translational energies, rovibrational levels (v, J) of HF, as well as initial parallel or perpendicular alignments between the HF rotational angular momentum and the reactants relative velocity vectors) have been performed on these PESs to compare them with experiments. The Sorbie-Murrell and LEPS-4 PESs lead to steric effect ratio results quite close to the experimental ones, once the error margins are included. The resllts point towards a bent K-F-H saddle point although the PES is very isotropic. This could explain why experimental determinations lead to suggest a collinear saddle point. The K + HF → KF + H reaction exhibits an enormous vibrational enhancement of reactivity with one quantum HF vibrational excitation, even at translational energies well above the HF(v=0) threshold, where tunnelling effect contribution to reactivity can be neglected. This behaviour has not been reproduced in the trajectory calculations and no satisfactory explanation has been obtained for this fact. Nevertheless, the HF(v=1)/HF(v=0) cross section ratio at translational energies not far from the HF(v=0) threshold and the relative cross section for HF(v=0) have been satisfactorily descibed. In what regards rotation, the best theoretical results are those corresponding to the Sorbie-Murrell PES (the cross section increases with J), although important differences with experiment appear for the J = 0–3 interval at the lower translational energy values considered (0.54 and 0.77 eV).     

Vibrational energies of PH3 calculated variationally at the complete basis set limit

The potential energy surface for the electronic ground state of PH(3) was calculated at the CCSD(T) level using aug-cc-pV(Q+d)Z and aug-cc-pVQZ basis sets for P and H, respectively, with scalar relativistic corrections included. A parametrized function was fitted through these ab initio points, and one parameter of this function was empirically adjusted. This analytical PES was employed in variational calculations of vibrational energies with the newly developed program TROVE. The convergence of the calculated vibrational energies with increasing vibrational basis set size was improved by means of an extrapolation scheme analogous to the complete basis set limit schemes used in ab initio electronic structure calculations. The resulting theoretical energy values are in excellent agreement with the available experimentally derived values.     

Photodissociation dynamics of the ethoxy radical investigated by photofragment coincidence imaging

The photodissociation dynamics of the ethoxy radical (CH3CH2O) have been studied at energies from 5.17 to 5.96 eV using photofragment coincidence imaging. The upper state of the electronic transition excited at these energies is assigned to the C2A'state on the basis of electronic structure calculations. Fragment mass distributions show two photodissociation channels, OH + C2H4 and CH3 + CH2O. The presence of an additional photodissociation channel, identified as D + C2D4O, is revealed in time-of-flight distributions from the photodissociation of CD3CD2O. The product branching ratios and fragment translational energy distributions for all of the observed mass channels are nonstatistical. Moreover, the significant yield of OH + C2H4 product suggests that the mechanism for this channel involves isomerization on the excited-state surface. Photodissociation at a much lower yield is seen following excitation at 3.91 eV, corresponding to a vibronic band of the B2A' <-- X2A' transition.     

Reactive scattering of H2 from Cu(100): six-dimensional quantum dynamics results for reaction and scattering obtained with a new, accurately fitted potential-energy surface

Six-dimensional quantum dynamical calculations are reported for the dissociative chemisorption of (v=0, 1, j=0) H(2) on Cu(100), and for rovibrationally inelastic scattering of (v=1, j=1) H(2) from Cu(100). The dynamics results were obtained using a new potential-energy surface (PES5), which was based on density-functional calculations using a slab representation of the adsorbate-substrate system and a generalized gradient approximation to the exchange-correlation energy. A very accurate method (the corrugation reducing procedure) was used to represent the density-functional theory data in a global potential-energy surface. With the new, more accurately fitted PES5, the agreement between the dynamics results and experimental results for reaction and rovibrationally elastic scattering is not as good as was obtained with a previous potential-energy surface (PES4), which was based on a subset of the density-functional theory data not yet including the results for the low-symmetry Cu sites. Preliminary density-functional theory results suggest that the agreement between theory and experiment will improve over that obtained with PES5 if the density-functional calculations are repeated using a larger basis set and using more copper layers than employed in PES4 and PES5.     

Guided-ion beam and theoretical study of the potential energy surface for activation of methane by W+

A guided-ion beam tandem mass spectrometer is used to study the reactions, W(+) + CH(4) (CD(4)) and [W,C,2H](+) + H(2) (D(2)), to probe the [W,C,4H](+) potential energy surface. The reaction W(+) + CH(4) produces [W,C,2H](+) in the only low-energy process. The analogous reaction in the CD(4) system exhibits a cross section with strong differences at the lowest energies caused by zero-point energy differences, demonstrating that this reaction is slightly exothermic for CH(4) and slightly endothermic for CD(4). The [W,C,2H](+) product ion reacts further at thermal energies with CH(4) to produce W(CH(2))(x)(+) (x = 2-4). At higher energies, the W(+) + CH(4) reaction forms WH(+) as the dominant ionic product with smaller amounts of WCH(3)(+), WCH(+), and WC(+) also formed. The energy dependent cross sections for endothermic formation of the various products are analyzed and allow the determination of D(0)(W(+)-CH(3)) approximately 2.31 +/- 0.10 eV, D(0)(W(+)-CH(2)) = 4.74 +/- 0.03 eV, D(0)(W(+)-CH) = 6.01 +/- 0.28 eV, and D(0)(W(+)-C) = 4.96 +/- 0.22 eV. We also examine the reverse reaction, [W,C,2H](+) + H(2) (D(2)) --> W(+) + CH(4) (CH(2)D(2)). Combining the cross sections for the forward and reverse processes yields an equilibrium constant from which D(0)(W(+)-CH(2)) = 4.72 +/- 0.04 eV is derived. Theoretical calculations performed at the B3LYP/HW+/6-311++G(3df,3p) level yield thermochemistry in reasonable agreement with experiment. These calculations help identify the structures and electronic states of the species involved and characterize the potential energy surface for the [W,C,4H](+) system.     

Interpolating moving least-squares methods for fitting potential energy surfaces: an application to the H2CN unimolecular reaction

Classical trajectories have been used to compute rates for the unimolecular reaction H2CN-->H+HCN on a fitted ab initio potential energy surface (PES). The ab initio energies were obtained from CCSD(T)/aug-cc-pvtz electronic structure calculations. The ab initio energies were fitted by the interpolating moving least-squares (IMLS) method. This work continues the development of the IMLS method for producing ab initio PESs for use in molecular dynamics simulations of many-atom systems. A dual-level scheme was used in which the preliminary selection of data points was done using a low-level theory and the points used for fitting the final PES were obtained at the desired higher level of theory. Classical trajectories were used on various low-level IMLS fits to tune the fit to the unimolecular reaction under study. Procedures for efficiently picking data points, selecting basis functions, and defining cutoff limits to exclude distant points were investigated. The accuracy of the fitted PES was assessed by comparing interpolated values of quantities to the corresponding ab initio values. With as little as 330 ab initio points classical trajectory rate constants were converged to 5%-10% and the rms error over the six-dimensional region sampled by the trajectories was a few tenths of a kcal/mol.