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The performance of the exchange-correlation density functional HCTH has been assessed using a benchmark of sulfur-containing molecules. Optimised structural parameters, harmonic frequencies and atomisation energies are presented and compared with calculations using the BLYP density functional, the MP2 methodology and appropriate experimental results. It is shown that, for sulfur compounds that do not contain halogens, the HCTH functional predicts geometries that are comparable to both the MP2 method and experiment, and harmonic frequencies are in much closer agreement with experiment than either the BLYP or MP2 methods; the atomisation energies are predicted within a few kcal mol-1 of the calculated MP2 results. The performance regarding sulfur-halogen compounds needs to be improved by the inclusion of F2S and Cl2S into the training set of future HCTH functionals.

The performance of numerical basis sets in relation to Gaussian basis sets is examined, by studying 20 small sulfur-containing molecules. The results of geometry optimization calculations are reported for each molecule using both density functional and Hartee-Fock methods. In comparison with experimental data, it is shown that the use of numerical bases tend to overestimate structural parameters, particularly bond lengths, and, in most cases, more than Gaussian basis sets. It is also shown that the use of a larger Gaussian basis set in DFT calculations has the effect of reducing bond lengths. © 1996 John Wiley & Sons, Inc.

An ab initio study including correlation corrections upto an MP3 or CISD level indicates that carbonyl imines are significantly higher in energy than the isomeric nitrones or oxaziridines, but that N —CN substitution reduces the energy difference substantially.

As the first part of a systematic theoretical study of oxacarbene rearrangement, ab initio SCF MO calculations employing a split valence shell basis set have been carried out for the model conversion of formaldehyde to the corresponding oxacarbene. The various cross‐sections of the reaction hypersurface were obtained by complete geometry optimization. The results suggest that, at least for the present model system, the reaction takes place preferentially in the first triplet (T1) state via a concerted pathway.

Ab initio calculations have been carried out in order to investigate the ground state (S O ) structure of ethylidene (CH 3 -CH). Results of the complete geometry optimization suggest that ethylidene has a classical staggered geometry. Attempted geometry of optimization of the cyclic “non-classical” alternative, suggested recently by Gordon et al. was unsuccessful, since it rapidly converges to the classical structure. Some aspects of the electronic mechanism of the rearrangement of ethylidene to ethylene are also discussed.