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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 is examined of the B3P86 hybrid exchange-correlation function for a set of 21 sulphur-containing molecules. Optimized geometries, harmonic frequencies and mean molecular polarizabilities are presented, and compared with calculations to the HartreeFock and MP2 levels of theory, and with appropriate experimental results. The hybrid functional predicts geometries at a level comparable with the MP2 results; harmonic frequencies are in closer agreement with experimental results than either MP2 or Hartree-Fock method; mean molecular polarizabilities predicted by the hybrid functional appear to be substantially closer to experiment than those calculated by either of the other two methods.

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.

A molecular dynamics study was undertaken to determine the conformational basis for the differing activities of the insect neuropeptide hormones calliFMRFamide 3 (SPSQDFMRF-NH2), calliFMRFamide 5 (APGQDFMRF-NH2) and their corresponding free-acid analogues (SPSQDFM- RF-OH and APGQDFMRF-OH) in two insect bioassays. A simulated annealing protocol was used to determine the range of conformers available to the linear peptides. Analysis of the conformers obtained indicated that all the peptides exhibited distinct secondary structure preferences. These, when correlated with their biological activities, enabled the formulation of putative conformation- activity relationships for the peptides.

A simple model of the photosynthetic unit has been constructed and used for simulated Forster-type energy migration, fluorescence and intersystem crossing, in order to gain insight into the conditions that influence both the form and the lifetime of the fluorescence decay in vivo. The model consists of a two-dimensional random lattice with one central trap. The simulation was done by means of repetitive Monte Carlo-type computations. The results obtained show that the form of the decay curve changes from exponential to non-exponential, as the chlorophyll concentration (molecules/nm2) is increased. The fluorescence lifetimes (tau 1/e) were also found to decrease substantially with only slight increases inc concentration. At a concentration comparable to that of chlorophyll in the chloroplast, both the form of the fluorescence decay and the lifetime are in fair agreement with experiment in vivo. The reasons for non-exponentially of the decay as well as the properties of energy migration are discussed. Preliminary work involving the dependence of trapping rate on donor concentration is also presented.

The effect of both the chlorophyll concentration and the chlorophyll-trap ratio on the trapping rate has been investigated by means of computer simulated energy migration on a two-dimensional random lattice. The rate equation for trapping was found to have the form: rate = 1.5 × 10 12 [donor] [trap] mole −1 nm 2 s −1 . Simulated fluorescence decay curves were also obtained at various concentrations. The calculated 1/e lifetimes were found to be in fair agreement with those obtained experimentally for in vivo systems.

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.

The energy profiles for the decomposition of the first triplet of acetaldehyde via the reactions: CH 3 CHO ν → CH 3 CO° + H°, CH 3 CHO * → CH . 3 + HCO . , have been computed using a non-empirical LCAO MO SCF procedure. Both reactions were found to have energy barriers of similar height (245 KJ mole −1 ) and at similar extensions of the respective rupturing bonds (0.71–0.76 A). The energy profile for internal rotation about the C-C bond was also calculated and showed a small barrier (3.5 kJ mole −1 ).

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.