A new hydrocarbon force field for saturated and non-conjugated unsaturated hydrocarbons has been developed. The most important difference between this force field and existing ones is its ability to produce a realistic, geometry-dependent charge distribution, the charges being calculated by the geometry-dependent method of Mortier (W. J. Mortier, S. K. Ghosh and S. Shankar, J. Am. Chem. SOC., 1986, 108, 4315). The use of this charge calculation means that polarization effects can be reproduced. Charge charge interactions are used between all the atoms in the molecules.Results show that by using this method a good hydrocarbon force field can be constructed. Heats of formation for a hundred compounds are calculated with an average absolute difference from experimental values of 1.02 kJ mol-' . Geometries, IR frequencies and conformational energies are also well reproduced. ~~ ~Force constants are given in kJ mol-' A-' rad-'. For valency angles containing two hydrogens no stretch-bend interaction was taken into account.

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A combination of the Metropolis Monte Carlo method and molecular mechanics calculations is used to study the Ti(IV) distribution in titanium silicalite-1 (TS-1). Calculations are carried out in which the Ti atoms are placed at crystallographically different T-sites rather than at symmetry-related T-positions. In this way, also the effect of already incorporated Ti atoms on the accommodation of new Ti atoms can be studied. It is shown that the Ti atoms are distributed over all crystallographically different lattice positions rather than located at one preferred T-site. The distribution, however, is not random:  T2 and T12 are clearly favored. Modeling of the Ti distribution at loadings above the experimentally determined maximum limit of 2/2 Ti atoms per unit cell seems questionable. The framework symmetry is related to both the location of the Ti atoms and the Ti loading. There are much less Ti atoms present as neighbors than can be expected from a random substitution, which might indicate that Loewenstein's rule is dependent not only on electrostatic effects but also on framework deformations.

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