The path of chemical reactions - the IRC approach

The reaction path on the potential energy surface of a polyatomic molecule is the steepest descent path (if mass‐weighted Cartesian coordinates are used) connecting saddle points and minima. For an N‐atom system in 3d space it is shown how the 3N‐6 internal coordinates can be chosen to be the reaction coordinate s, the arc length along the reaction path, plus (3N‐7) normal coordinates that describe vibrations orthogonal to the reaction path. The classical (and quantum) Hamiltonian is derived in terms of these coordinates and their conjugate momenta for the general case of an N atom system with a given nonzero value of the total angular momentum. One of the important facts that makes this analysis feasible (and therefore interesting) is that all the quantities necessary to construct this Hamiltonian, and thus permit dynamical studies, are obtainable from a relatively modest number of ab initio quantum chemistry calculations of the potential energy surface. As a simple example, it is shown how the effects o...

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Reaction path Hamiltonian for polyatomic molecules

With recent advances in electronic structure methods, first-principles calculations of electronic response properties, such as linear and nonlinear polarizabilities, have become possible for molecules with more than 100 atoms. Basis set incompleteness is typically the main source of error in such calculations since traditional diffuse augmented basis sets are too costly to use or suffer from near linear dependence. To address this problem, we construct the first comprehensive set of property-optimized augmented basis sets for elements H–Rn except lanthanides. The new basis sets build on the Karlsruhe segmented contracted basis sets of split-valence to quadruple-zeta valence quality and add a small number of moderately diffuse basis functions. The exponents are determined variationally by maximization of atomic Hartree–Fock polarizabilities using analytical derivative methods. The performance of the resulting basis sets is assessed using a set of 313 molecular static Hartree–Fock polarizabilities. The mean...

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Property-optimized gaussian basis sets for molecular response calculations.

We report on the determination of a high quality ab initio potential energy surface (PES) and dipole moment function for water This PES is empirically adjusted to improve the agreement between the computed line positions and those from the HITRAN 92 data base with J⩽5 for H216O The changes in the PES are small, nonetheless including an estimate of core (oxygen 1s) electron correlation greatly improves the agreement with the experiment Using this adjusted PES, we can match 30 092 of the 30 117 transitions in the HITRAN 96 data base for H216O with theoretical lines The 10, 25, 50, 75, and 90 percentiles of the difference between the calculated and tabulated line positions are −011, −004, −001, 002, and 007 cm−1 Nonadiabatic effects are not explicitly included About 3% of the tabulated line positions appear to be incorrect Similar agreement using this adjusted PES is obtained for the 17O and 18O isotopes For HD16O, the agreement is not as good, with a root-mean-square error of 025 cm−1 for line

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The determination of an accurate isotope dependent potential energy surface for water from extensive ab initio calculations and experimental data

The nature of the bonding in transition-metal compounds.

Editorial Board: W. G. E. Caldwell, OC, FRSC (University of Western Ontario); K. G. Davey, OC, FRSC (York University); S. Gubins (Annual Reviews); B. K. Hall, FRSC (Dalhousie University); P. Jefferson (Agriculture & Agri-Food Canada); W. H. Lewis (Washington University); A. W. May, OC (Memorial University of Newfoundland); N. R. Morgenstern, CM, AOL, FRSC (University of Alberta); B. P. Dancik, Editor-in-Chief, NRC Research Press (University of Alberta)

Molecular symmetry and spectroscopy

The vibration-rotation problem in triatomic molecules allowing for a large-amplitude bending vibration

Partial table of contents: The Born--Oppenheimer Approximation (P. Bunker & P. Jensen). ELECTRONIC STATES. Ab Initio Determination of Accurate Ground Electronic State Potential Energy Hypersurfaces for Small Molecules (A. Csaszar, et al.). Symmetry Adapted Perturbation Theory Applied to the Computation of Intermolecular Forces (R. Moszynski, et al.). The Ab Initio Calculation of Molecular Properties Other than the Potential Energy Surface (S. Sauer & M. Packer). ROTATION--VIBRATION STATES. Perturbation Theory, Effective Hamiltonians and Force Constants (K. Sarka & J. Demaison). Variational Calculations of Rotation--Vibration Spectra (J. Tennyson). ROVIBRONIC STATES AND THE BREAKDOWN OF THE BORN--OPPENHEIMER APPROXIMATION. The Renner Effect (P. Jensen, et al.). The Renner--Teller Effect: The Effective Hamiltonian Approach (J. Brown). DYNAMICS. Forming Superposition States (T. Seideman). Ab Initio Molecular Dynamics (J. Tse & R. Rousseau). Index.

Computational Molecular Spectroscopy

We have observed and assigned a number of far infrared laser magnetic resonance spectra of CH2 arising from rotational transitions within the lowest vibrational state of the a 1A1 electronic excited state and from transitions between such singlet levels and vibrationally excited levels of the X 3B1 electronic ground state The singlet–singlet transitions are magnetically active, and the singlet–triplet transitions have electric dipole intensity because of the spin‐orbit mixing of singlet levels with vibrationally excited levels of the triplet state By identifying four pairs of singlet and triplet levels that perturb each other we can accurately position the singlet and triplet state relative to each other and determine the single–triplet energy splitting We determine that T0(a 1A1)=3165±20 cm−1 (905±006 kcal/mol; 0392±0003 eV), and Te(a 1A1)=2994±30 cm−1 (856±009 kcal/mol; 0371±0004 eV) A new ab initio calculation of the spin‐orbit matrix element between these two states has been of assistance in assigning the levels that perturb each other and has enabled us to calculate the radiative lifetimes of the lowest ortho and para levels of the a 1A1 state to be about 18 s in each case

Far infrared laser magnetic resonance of singlet methylene: Singlet–triplet perturbations, singlet–triplet transitions, and the singlet–triplet splittinga)

Philip R. Bunker

Papers

Molecular symmetry and spectroscopy

Molecular symmetry and spectroscopy

The vibration-rotation problem in triatomic molecules allowing for a large-amplitude bending vibration

Computational Molecular Spectroscopy

Far infrared laser magnetic resonance of singlet methylene: Singlet–triplet perturbations, singlet–triplet transitions, and the singlet–triplet splittinga)