Donald R. McLaughlin

Interatomic correlation energy and the van der Waals attraction between two helium atoms

Abstract A priori calculations have been carried out on He-He for 26 internuclear separations between 3.0 and 10.0 bohrs. The interatomic correlation energy is added to the Hartree-Fock energy to predict a potential energy curve. The calculated potential parameters are ϵ = 12.02°K, σ = 2.608 A, R e = 2.944 A. Predicted values of the coefficients C 6 and C 8 are within 7% and 3% of the accepted values. The interatomic correlation energy is divided into contributions from atomic and molecular orbitals of different symmetry types.

Ab initio dynamics: HeH+ + H2 → He + H3+ (C2ν) classical trajectories using a quantum mechanical potential‐energy surface

Tabular values of near‐Hartree‐Fock ab initio energies for the ground electronic state of HeH3+ were expressed in analytical form by fitting with spline interpolation functions. The interpolation functions were then used to provide the potential‐energy gradients necessary for dynamical calculations. General features of this method for representing potential‐energy surfaces are discussed. Classical trajectories have been computed for the ion‐molecule reaction HeH++H2 → He+H3++2.6 eV, constrained to follow C2ν symmetry reaction paths for which there is no energy barrier. Trajectories were run under the conditions: relative translational energies 0.001 and 0.1298 eV and internal vibrational states of reactants with quantum numbers 0 and 2; the initial vibrational phases of each trajectory were selected randomly. Analysis of the results revealed that most of the energy available to the products (including the exothermicity of the reaction) ends up as H3+ vibrational energy; more than half of the collisions pr...

A theoretical prediction of vibrational enhancement for dissociative charge transfer in the HeH2+ system

A model has been developed to treat dissociative charge exchange occurring on excited‐state surfaces in the weak adiabatic limit. This process is analogous to collisionally induced predissociation with the added feature that electronic as well as translational energy is used to effect the dissociation. The consequences of weak adiabaticity and the location of the avoided crossing seam are to require the consideration of tunnelling through a barrier in the dissociation curve. An appealing feature of the model is the reduction of a three‐dimensional trajectory problem to one dimension by neglect of coupling of translation to vibration and rotation. Configuration interaction ground‐ and excited‐state calculations have been performed in the vicinity of the curve crossing seam for He++H2→He+H++H. Use of the parameters derived from these ab initio results in the dynamics model yields cross section and rate coefficient values in qualitative agreement with experimental measurements. Vibrational excitation from th...

Geometry and force constant determination from correlated wave functions for polyatomic molecules: Ground states of H2O and CH2

Ab initio calculations including electron correlation are reported for the water and methylene molecules as a function of geometry. A large contracted gaussian basis set is used and the multiconfiguration wave functions, optimized by the iterative natural orbital procedure, include 277 and 617 configurations for H2O and CH2 respectively. The method of selecting configurations, yielding “first-order” wave functions, is discussed in some detail. For H2O, the SCF geometry is r=0,942 Å, θ=105,8°, the correlated result is r=0,968 Å, θ=103,2°, and the experimental r=0,957 Å, θ=104,5°. The water stretching force constants, in millidynes/Å, are 8,72 (SCF), 8,75 (CI), and 8,4 (experiment). Bending force constants are 0,88 (SCF), 0,83 (CI), and 0,76 (experiment). For methylene the SCF geometry is r=1,072 Å, θ=129,5°, while the result from first-order wave functions is r=1,088 Å, θ=134°. The predicted CH2 force constants are 6,16 (SCF) and 6,13 (CI) for stretching and 0,44 (SCF) and 0,33 (CI) for bending.ZusammenfassungEs wird über ab intito-Rechnungen mit Berücksichtigung der Elektronenkorrelation berichtet, die an Wasser- und Methylenmolekülen als Funktion der Geometrie durchgeführt worden sind. Dazu benutzt man einen großen kontrahierten Gauß-Basissatz. Die Multikonfigurationswellenfunktionen, die unter Benutzung von natürlichen Orbitalen nach der iterativen Prozedur optimiert werden, enthalten für H2O 277 Konfigurationen und für CH2 617. Die Auswahlmethode, die zu Wellenfunktionen 1. Ordnung führt, wird diskutiert. Im Falle des Wassers erhält man die SCF-Geometrie zu r=0,942 Å, θ=105,8°, das korrelierte Resultat ist: r=0,968 Å, θ=103,2° und das experimentelle r=0,957 Å, θ=104,5°. Für Wasser ergeben sich die Valenzkraftkonstanten (in Millidyn Å−1) 8,72 (SCF), 8,75 (CI) und 8,4 (Experiment). Die Deformationskonstanten sind 0,88 (SCF), 0,83 (CI) und 0,76 (Experiment). Im Falle des Methylens ist die SCF-Geometrie r=1,072 Å, θ=129,5°, während man mit Wellenfunktionen 1. Ordnung r=1,088 Å und θ=134° erhält. Die CH2-Kraftkonstanten werden für die Valenzschwingung zu 6,16 (SCF) und 6,13 (CI) bzw. für die Deformationsschwingung zu 0,44 (SCF) und 0,33 (CI) vorausgesagt.

Quantum Mechanical Investigation of the HeH3+ Energy Surface

Results of an ab initio calculation of the potential surface of HeH3+ are reported, with emphasis on establishing the low energy reaction paths for HeH+ + H2 = He + H3+ within the single configuration molecular orbital (SCF) approximation. Using a basis set of (9s1p/4s1p) Gaussian functions centered on the (He/H) nuclei, the lowest potential energy paths are found to follow C2v symmetry, and these are examined in greater detail. A small number of configuration interaction (CI) calculations are reported involving all single and double excitations from the single configuration. From these calculations about 80% of the total correlation energy (or 90% of the p limit correlation energy) is obtained for the separated two electron fragments, and the CI surface is indicated to lie parallel to the SCF surface to within a few percent of the total energy change of the reaction. The C2v minimal energy reaction path is found to decrease monotonically with no reaction barriers or local minima, with the heat of reactio...

A quasiclassical trajectory study of the H2+F2 reactions

A theoretical investigation of the reaction dynamics of the reactions H2(v=0,2,4, or 6) + F2(v=0 or 6) → H+HF+F, 2HF, F2H+H, H2F+F, F2+2H, and H2+2F has been accomplished using Monte Carlo averaging over quasiclassical trajectories on a reasonable semiempirical potential‐energy surface. The details of the dynamical mechanism of the reaction processes were investigated by examining geometrically restricted reaction paths and selected restricted initial states. Reaction cross sections have been computed and estimates of rate coefficients have been made. The computed thermal rates are many orders of magnitude lower than thermal collision frequencies. Reaction thresholds are lowered significantly for vibrationally excited hydrogen.

Temperature effects on the carbon-13 NMR shifts of selected compounds

Abstract The effects of temperature on the 13 C NMR shifts of benzene, cyclohexane, pyridine, cyclohexene, neopentane, hexane, and heptane have been determined from +10 to +70°C. In addition, the temperature shift of 2 D in D 2 O was found to be +10.4 × 10 −3 ppm/°C. All shifts appear to be linear functions of temperature within experimental error.

An Improved Variational Calculation of the Lower Vibrational Energy Levels of the Ammonia Molecule

Abstract A new variational calculation of the inversion spectrum of ammonia is reported in which all six vibrational degrees of ammonia are employed. By fitting spectroscopic data, an inversion barrier of 1810 cm-1 is obtained.

Variational Calculation of the Lower Vibrational Energy Levels of the Ammonia Molecule

Abstract A variational calculation of the lower-lying vibrational energy levels of the ammonia molecule is performed, in which all six vibrational degrees of freedom are treated simultaneously. A literature potential is assumed for all non-inversion motions; a new inversion barrier potential with a barrier of 1616 cm-1 is determined by fitting experimental data.

Symbolic Computation in Chemical Education

The past two decades have witnessed revolutions in computing techniques used by students at all levels of science education. The slide rule has taken its place beside the abacus as a historical curiosity, and the electronic pocket calculator is now edging past the computing power of the first “giant electronic brains”. The end of the computer age is not yet in sight, and we are about to witness another equally impressive decade of developments which will most likely render today’s calculational methods obsolete.