IL13. Proton-Bound Complexes: A Major Challenge for Vibrational Analysis

I will review a general approach to develop and employ full-dimensional potential and dipole moment surfaces in VSCF/VCI calculations of highly-coupled vibrational motion in proton-bound complexes. Examples include the proton-bound N2 [1] and CO[2] dimers and proton-bound water clusters, H7O4+ and H9O4+.[3] A controversy about the latter “Eigen” complex will be addressed, as it points out some limitations of “ab initio molecular dynamics” approaches to IR spectroscopy

Exact quantum quasiclassical, and semiclassical reaction probabilities for the collinear F+D_2 → FD+D reaction

Exact quantum, quasiclassical, and semiclassical reaction probabilities and rate constants for the collinear reaction F+D_2 → FD+D are presented. In all calculations, a high degree of population inversion is predicted with P^R_(03) and P^R(04) being the dominant reaction probabilities. In analogy with the F+H_2 reaction (preceding paper), the exact quantum 0→3 and 0→4 probabilities show markedly different energy dependence with PR03 having a much smaller effective threshold energy (E_T=0.014 eV) than P^R_(04) (0.055 eV). The corresponding quasiclassical forward probabilities P^R_(03) and P^R_(04) are in poor agreement with the exact quantum ones, while their quasiclassical reverse and semiclassical counterparts provide much better approximations to the exact results. Similar comparisons are also made in the analysis of the corresponding EQ, QCF, QCR, and USC rate constants. An information theoretic analysis of the EQ and QCF reaction probabilities indicates nonlinear surprisal behavior as well as a significant isotope dependence. Additional quantum results at higher energies are presented and discussed in terms of threshold behavior and resonances. Exact quantum reaction probabilities for the related F+HD → FH+D and F+DH → FD+H reactions are given and an attempt to explain the observed isotope effects is made

Large quantum effects in the collinear F+H2-->FH+H reaction

We have performed accurate quantum mechanical calculations of reaction probabilities for the collinear F+H2-->FH+H reaction as well as corresponding quasiclassical trajectory calculations. A comparison of these results shows that very significant quantum mechanical effects are present in this reaction

Dynamics of the O(3P) + CHD3(vCH = 0,1) reactions on an accurate ab initio potential energy surface

Recent experimental and theoretical studies on the dynamics of the reactions of methane with F and Cl atoms have modified our understanding of mode-selective chemical reactivity. The O + methane reaction is also an important candidate to extend our knowledge on the rules of reactivity. Here, we report a unique full-dimensional ab initio potential energy surface for the O((3)P) + methane reaction, which opens the door for accurate dynamics calculations using this surface. Quasiclassical trajectory calculations of the angular and vibrational distributions for the ground state and CH stretching excited O + CHD(3)(v(1) = 0,1) → OH + CD(3) reactions are in excellent agreement with the experiment. Our theory confirms what was proposed experimentally: The mechanistic origin of the vibrational enhancement is that the CH-stretching excitation enlarges the reactive cone of acceptance

Exact quantum, quasiclassical, and semiclassical reaction probabilities for the collinear F+H2 --> FH+H reaction

Exact quantum, quasiclassical, and semiclassical reaction probabilities and rate constants for the collinear reaction F+H2 --> FH+H are presented and compared. The exact quantum results indicate a large degree of population inversion of the FH product with PR02 and PR03 being the dominant reaction probabilities. The energy dependence of these two probabilities at low translational energies are quite different. PR02 shows an effective threshold of 0.005 eV which can largely be interpreted as resulting from tunneling through a vibrationally adiabatic barrier. PR03 has a much larger effective threshold (0.045 eV) apparently resulting from dynamical effects. Quasiclassical probabilities for the collinear F+H2 reaction were calculated by both the forward (initial conditions chosen for reagent F+H2) and reverse (initial conditions for product H+FH) trajectory methods. The results of both calculations correctly indicate that PR03 and PR02 should be the dominant reaction probabilities. However, the threshold behavior of the quasiclassical forward PR03 disagrees strongly with the corresponding exact quantum threshold energy dependence. By contrast, there is good agreement between the reversed trajectory results and the exact quantum ones. The uniform semiclassical results also agree well with the corresponding exact quantum ones indicating that the quasiclassical reverse and the semiclassical methods are preferable to the quasiclassical forward method for this reaction. The important differences between the threshold behavior of the exact quantum and quasiclassical forward reaction probabilities are manifested in the corresponding rate constants primarily as large differences in their activation energies. Additional exact quantum results at higher total energies indicate that threshold effects are no longer important for reactions with vibrationally excited H2. Resonances play an important role in certain reaction probabilities primarily at higher relative translational energies

Theoretical studies of electronically adiabatic and non-adiabatic chemical reaction

Part I presents several sets of comparisons of semi-classical, quasi-classical and exact quantum reactive scattering calculations for collinear chemical reactions. The possibility of modifying the standard quasi-classical method according to a quantum criterion is investigated. The systems studied are H + H_2, F + H_2, and F + D_2. In addition, a theoretical investigation of the semi-classical S matrix is made. Details of a quasi-classical current density analysis of the H + H_2 reaction are presented and a comparison with exact quantum results is made. A direct test of two versions of the vibrationally adiabatic theory of chemical reactions is made in Part II for the H + H_2 reaction. The adiabaticity of the symmetric stretch motion of the H_3 transition state is focussed upon. In addition, a determination of the completeness of adiabatic basis sets for scattering calculations is made. The theory of electronically non-adiabatic chemical reactions is presented in Part III. Quantum calculations of the collinear H^+ + H_2 → H_2 + H^+ reaction are described. A model and a realistic potential energy surface are employed in these calculations. A fictitious electronically non-adiabatic H + H_2 collinear chemical reaction is treated quantum mechanically. Two potential energy surfaces and a coupling surface are developed for this purpose. The reaction Ba(^1S) + ON_2(X^1Σ) → BaO(X^1Σ) + N_2(X^1Σ^+_g), BaO(a^3II) + N_2(X^1Σ^+_g) is studied quantum mechanically. The singlet and triplet potential energy surfaces are devised as is a spin-orbit coupling surface. Electronically adiabatic and non-adiabatic transition probabilities are calculated as a function of the initial translational energy of the reagents.</p

Full-dimensional (15-dimensional) ab initio analytical potential energy surface for the H 7 + cluster

Full-dimensional ab initio potential energy surface is constructed for the H7+ cluster. The surface is a fit to roughly 160 000 interaction energies obtained with second-order MöllerPlesset perturbation theory and the cc-pVQZ basis set, using the invariant polynomial method [B. J. Braams and J. M. Bowman, Int. Rev. Phys. Chem. 28, 577 (2009)10.1080/01442350903234923]. We employ permutationally invariant basis functions in Morse-type variables for all the internuclear distances to incorporate permutational symmetry with respect to interchange of H atoms into the representation of the surface. We describe how different configurations are selected in order to create the database of the interaction energies for the linear least squares fitting procedure. The root-mean-square error of the fit is 170 cm -1 for the entire data set. The surface dissociates correctly to the H5+ H 2 fragments. A detailed analysis of its topology, as well as comparison with additional ab initio calculations, including harmonic frequencies, verify the quality and accuracy of the parameterized potential. This is the first attempt to present an analytical representation of the 15-dimensional surface of the H7+ cluster for carrying out dynamics studies. © 2012 American Institute of Physics.. R.P. acknowledges financial support by Ministerio de Ciencia e Innovacion, Spain, Grant No. FIS2010-18132, FIS2011-29596-C02-01 and European Cooperation in Scientific and Technical Research (COST) Action CM1002 (CODECS). P.B. acknowledges a postdoctoral fellowship from the Ramón Areces Foundation, Spain. J.M.B. and Y.W. thank the National Science Foundation (CHE-1145227) for financial support.Peer Reviewe