Peter G. Burton

Ab initio calculation of near‐equilibrium potential and multipole moment surfaces and vibrational frequencies of H+3 and its isotopomers

H+3 potential energies and multipole moments are calculated from a full CI with a 10s, 4p, 2d GTO hydrogen basis. 69 calculated energy points with energies of up to 25 000 cm−1 above the minimum are fitted by a power series expansion in terms of a Morse‐type coordinate with a mean square error of less than 1 cm−1. Rotationless vibrational states with energies of up to 12 000 cm−1 above equilibrium are calculated variationally for ten isotopomers. The resulting band origins for the seven analyzed fundamental transitions show a mean deviation of less than 2 cm−1. For the other predicted frequencies, the errors are expected to be below 0.1% also. The equilibrium bond length of H+3 is predicted to be 0.8732(2) A.

The (H2)2 potential surface and the interaction between hydrogen molecules at low temperatures

The (H2)2 rigid rotor interaction potential has been calculated for the intermolecular distance range R = 3.0–11.0 a.u. for six relative orientations to estimate both the isotropic and anisotropic components of the full intermolecular potential. A partially optimized basis set limited in size to 78 independent Gaussian functions was used throughout the energy calculations, which required only very small corrections for basis set unsaturation effects. Correlation effects were computed both at the variational (single and) double excitation PNOCI level and using the CEPA2‐PNO approximation to estimate higher order excitation effects. While the latter rigid‐rotor surface may overestimate the strength of the H2–H2 interaction in the vicinity of the well by a few wave numbers in the rigid‐rotor PNOCI surface from the present study, which we regard as an upper bound to the true rigid‐rotor surface, is also slightly deeper than almost all previous theoretical and empirical ’’fit’’ potentials in the well region. S...

The cyclic ozone isomer

Analysis of basis set effects in large correlated wave function calculations is undertaken in relation to the energy separation of the cyclic (D3h) and open (C2v) forms of ozone. The present calculations attain some 60% of the valence correlation energy of ozone with the largest bases, superseding a number of recent investigations of the D3h−C2v separation. The conclusion is reached that these species are isomers corresponding to different local minima on the ground state surface, below the ozone dissociation limit. The energy separation of these species is narrowed down to 40–88 kJ/mol, with the likeliest value, 50±kJ/mol, emerging from the present calculations (’’doulbe zeta plus polarization’’ is in error ∼25 kJ/mol). The formation and implications of the secondary D3h ozone isomer are briefly discussed.

The vibration spectrum of H3

The H3 + ab initio potential energy surface and its analytical representation has been re-examined to promote more accurate vibration-rotation calculations. We present here a new and accurate 78 point PNO-CI grid which predicts the minimum energy geometry to be an equilateral triangle of side, r (H-H) = 1·6525 a 0. and energy of -1·34188 E h. Of the analytical representations only the sixth order Simons-Parr-Finlan (SPF) and Ogilvie force fields were found to satisfy our fitting criteria, resulting in associated errors of less than 11·1 kJ mol-1. Our calculated vibrational band origins are in good agreement with those of Carney and Porter, with the differences of the lowest-lying vibration states essentially reflecting the analytical characteristics of the potential energy surfaces used.

Comparison of perturbatively corrected MRD CI results with a full CI treatment of the BH ground state

Abstract It is demonstrated by direct comparison with a full CI calculation for BH (DZ basis at r e ) that the corresponding energy value can be obtained to high accuracy (0.01 kJ/mol −1 ) by employing extrapolation from a series of MRD CI treatments which consider at most a small fraction of the total configuration space (order 17049 versus 132686). The perturbative methods employed are of quite general applicability and are seen to provide high overall accuracy in an economically feasible manner.

Design of basis sets for precise intermolecular force computation. Investigation of the He2 potential curve using CEPA‐PNO correlated wavefunctions.

Techniques for the systematic improvement of atomic (or molecular) basis sets for precise intermolecular force computations are used to refine the theoretical He2 ground state potential curve. Diagnostic parameters from the most recent experimental study of this curve (re=5.61a0, De=10.57 °K) are matched in the present work (re=5.63a0, De=10.55 °K). Specifications for the minimum basis capable of yielding a reliable He2 potential curve are elucidated, opening the way for a definitive determination of this curve from first principles.

A CEPA2 investigation of the He-He and He-Li+ potential functions

An ab initio investigation of the He-He and He-Li+ ground state potential functions has been carried out using the CEPA2 formalism. A large gaussian basis set, optimised for the He-He interaction, was used to calculate the potentials from R = 1.0 a 0 to 15.0 a 0. Two electron integrals generated in the He-He calculation were used in the computation of the He-Li+ interaction. For He-He the well depth was found to be 9.55 K at R = 5.67 a 0. The He-Li+ well depth was determined to be 2.955 mE h† at R = 3.58 a 0.

Bond functions in SCF calculations on molecules containing second-row atoms

Basis set expansion in molecules containing second-row atoms by inclusion of s and p-type gaussian functions centred on the bond axes is shown to be efficient and is compared with the addition of d-orbitals to the basis set, in the test molecule SO2. Calculations on SO2 and other molecules at MBS and double-zeta level have been carried out with wave-function polarization introduced by bond functions at various locations in the molecule. Some simple rules for basis set enhancement by this method are presented.

Bond function parameter transferability in SO2, SF2 and SO2F2

Ab initio MO studies of the use of gaussian bond functions in the molecules SO2,SF2 are presented. It is shown that optimization of bond function exponent and position parameters in the small molecules is a good approximation to the optimum parameter set for SO2F2,

The vibrational spectra of H3+ and D3+: Improved values from new representations of AB initio surfaces

Results for the vibrational frequencies of H3+ and D3+ are calculated using the potential surfaces of Burton, von Nagy-Felsobuki, Doherty and Hamilton (BVDH), and Dykstra and Swope (DS). The νE fundamental frequencies from these potentials for H3+ are 2519.29 and 2521.62 cm−1 respectively (exp. 2521.6 cm−1). For D3+ the values are 1831.07 and 1834.7 cm−1 (exp. 1835.3 cm−1). The analytical fit used here for the BVDH surface is an improvement on previously reported fits for this surface in that the low-lying part of the well is emphasized. The present results confirm the relative accuracy and shape of the surface mapped out by the electronic energies reported by BVDH.