The thermodynamic and spectroscopic properties of the oxygen fluoride species have been reviewed. Recommended thermochemical tables are given for five gaseous oxygen fluorides: OF, OFO, FOO, FOF, and O2F2. Sufficient information is not available to generate thermochemical tables for any condensed phase species. Annotated bibliographies (over 600 references) are provided for all neutral oxygen fluorides which have been reported in the literature. There are needs for additional experimental and theoretical data to reduce the uncertainties in the recommended values for these five species. Of all the species mentioned in the literature, many have not been isolated and characterized. In fact, some do not exist. Throughout this paper, uncertainties attached to recommended values correspond to the uncertainty interval, equal to twice the standard deviation of the mean.

NIST‐JANAF Thermochemical Tables for Oxygen Fluorides

A method is presented for approximating multicenter integrals which arise in LCAO MO calculations utilizing Slater‐type orbitals as basis functions. Two‐center charge distributions are projected onto one‐center functions, allowing all difficult three‐ and four‐center integrals to be expressed as linear combinations of rapidly calculated one‐ and two‐center Coulomb integrals. Test calculations are reported for a wide variety of polyatomic molecules yielding excellent agreement with corresponding ab initio treatments. Integral generation rates are well over an order of magnitude faster than current accurate approaches. The systematic and reliable nature of the approximation is examined and comparison made with the recently reported PDDO (projection of diatomic differential overlap) technique.

Limited Expansion of Diatomic Overlap (LEDO): A Near‐Accurate Approximate Ab Initio LCAO MO Method. I. Theory and Preliminary Investigations

On the justifiability of neglect of differential overlap molecular orbital methods

Semiempirical molecular orbital (SEMO) models based on the neglect of diatomic differential overlap (NDDO) approximation efficiently solve the self-consistent field equations by rather drastic approximations. The computational efficiency comes at the cost of an error in the electron-electron repulsion integrals. The error may be compensated by the introduction of parametric expressions to evaluate the electron-electron repulsion integrals, the one-electron integrals, and the core-core repulsion. We review the resulting formalisms of popular NDDO-SEMO models (such as the MNDO(/d), AM1, PMx, and OMx models) in a concise and self-contained manner. We discuss the approaches to implicitly and explicitly describe electron correlation effects within NDDO-SEMO models and we dissect strengths and weaknesses of the different approaches in a detailed analysis. For this purpose, we consider the results of recent benchmark studies. Furthermore, we apply bootstrapping to perform a sensitivity analysis for a selection of parameters in the MNDO model. We also identify systematic limitations of NDDO-SEMO models by drawing on an analogy to Kohn--Sham density functional theory.

Semiempirical molecular orbital models based on the neglect of diatomic differential overlap approximation

Self‐broadened and foreign‐gas (N2 and O2) broadened linewidths of O3 at 300 °K for a wide range of quantum numbers J and Ka, for both type A and type B bands, have been calculated using the Anderson–Tsao–Curnutte theory of line broadening. In the case of O3–O3 collisions, dipole–dipole, dipole–quadrupole, quadrupole–dipole, and quadrupole–quadrupole interactions have been included. For O3–N2 and O3–O2 collisions, dipole–quadrupole and quadrupole–quadrupole interactions were taken into account. Good agreement was obtained between the available measured linewidths and the computed values for corresonding transitions on using the molecular quadrupole moments of O3 given by Rothenberg and Schaefer. Air‐broadened linewidths of O3 at 200 °K have also been computed, so that the temperature dependence can be established.

Pressure-broadened linewidths of ozone

A method is described for reducing a large part of the arithmetic of exact ab-initio SCF molecularorbital calculations based on Slater-type-orbitals without noticeable loss of numerical accuracy. The procedure involves the transformation to Lowdin orthogonalized orbitals and then invoking the NDDO approximation. The three- and four-centre two-electron integrals required are estimated by a truncated Ruedenberg expansion. All one-electron integrals are evaluated exactly. No empirical parameters are employed. Numerical tests on CO, OF2, O3 and ONF show that the NDDO approximation is very accurate for Lowdin functions and that the Ruedenberg expansion is arithmetically satisfactory for the SCF MO calculations.

Simplified ab-initio calculations for molecular systems

The simplified ab-initio method described in an earlier paper is tested on some hydrogen-containing molecules. The performance is slightly below that found previously for molecules composed entirely of first-row atoms but should be suitable for applications where limited numerical accuracy is sufficient. The hope of improved performance through limited expansion of the basis, especially on hydrogen, is not realised and so alternative treatments of the two-electron many-centre integrals should be sought if greater numerical accuracy is required.

Simplified ab-initio calculations on hydrogen-containing molecules

A study has been made of the way in which the calculated positions of excited states of non-alternant hydrocarbons are affected by (a) including all resonance integrals rather than just those for nearest neighbours (b) including configuration-interaction. The VESCF molecular-orbital technique, with two alternative methods for deriving basic integrals, was employed. Predicted positions of excited states are found to be sometimes appreciably altered when all resonance integrals are included, and are somewhat dependent upon the method of evaluation of basic integrals. We conclude that one should be cautious in interpreting U.V. spectra from an isolated molecular-orbital calculation on a non-alternant hydrocarbon. The study of the effect of configuration-interaction for the excited states of fulvene and dimethylenecyclobutene yields results analogous to those found by Koutecky and co-workers for benzene. The predicted relative positions of some excited states are altered when doubly excited configurations are included, but the general level of energies predicted for lower excited states when only singly excited configurations are used is probably close to the result that would be found when up to triply excited configurations are included.

Ultraviolet spectra of non-alternant hydrocarbons: The significance of non-neighbour resonance integrals and of configuration-interaction

Ab initio unrestricted Hartree-Fock (UHF) calculations (with and without single annihilation) have been performed on the radical NF2 using four different basis sets; namely, a minimal basis Slater set, two minimal basis Gaussian sets and a Gaussian set of approximately double zeta accuracy. Several one-electron charge dependent properties have been calculated with each basis set and it is most apparent that near double zeta accuracy 2p functions are necessary to produce reliable values. Single annihilation of the UHF wavefunctions calculated with the two basis sets containing near double zeta 2p functions, was found to give an accurate representation of the anisotropic coupling constants at both the nitrogen and fluorine atoms. Less satisfactory agreement with the experimental isotropic coupling constants was found with all calculations. This investigation of the NF2 radical indicates that, providing a good quality basis set is used, the single annihilated UHF method can provide accurate values for most charge and spin dependent observable properties of open shell molecules.

The electronic structure of the NF2 radical

An ab-initio SCFMO technique of balanced computational speed and precision is described for use with Slater-type orbitals as basis. All of the more significant integrals over STO'S are evaluated with full arithmetic accuracy but the ?non-NDDO? two-electron integrals are evaluated less precisely by using small Gaussian expansions. The overall numerical accuracy achieved in calculations of total energy and are illustrated for a number of small molecules using both 2G and 3G expansions. The performance, especially with respect to energy calculations, is clearly superior to that of analogous methods that have recently been described.

G. R. Williams

Papers

Simplified ab-initio calculations for molecular systems

Simplified ab-initio calculations on hydrogen-containing molecules

Ultraviolet spectra of non-alternant hydrocarbons: The significance of non-neighbour resonance integrals and of configuration-interaction

The electronic structure of the NF2 radical

Non-empirical molecular orbital calculations based on Slater-type orbitals. A restricted use of Gaussian expansions