P.T. van Duijnen

Mean polarizabilities of organic molecules. A comparison of restricted Hartree Fock, density functional theory and direct reaction field results

Abstract The polarizabilities of 15 organic molecules are calculated using the Restricted Hartree Fock (RHF) method, the Time Dependent Density Functional Theory (TD-DFT) and the Direct Reaction Field (DRF) approach. The RHF method gives rather poor results, while the other two give average deviations comparable to the experimental uncertainty. The DRF approach is very fast (

Many-body interactions calculated with the direct reaction field model

Abstract The direct reaction field (DRF) model was developed for calculations of electronic properties of molecules in the condensed phase. In the DRF approach the electrons of (part(s) of a system is described with wavefunctions, the larger parts classically with point charges and polarizabilities. Leaving out the quantum-mechanical part(s) leads naturally to a polarizable force field. In this paper we demonstrate the usefulness of the DRF model for the study of many-body interactions in polar systems. We have calculated the many-body interactions in clusters of HF, H2O and urea in our classical polarization model and compared the results to ab initio calculations using large basis sets. We find that the results obtained using the classical model compare excellently to ab initio results.

Some applications of the direct reaction field approach

The Direct Reaction Field approach is briefly reviewed. Preliminary reports of the calculations on solvent induced shifts in the p * ˆ n transition of acetone in various solvents, and the dissociation of tert-butyl chloride in water are given. q 1999 Elsevier Science B.V. All rights reserved.

Substituent effects on gas-phase acidities of formic acid and its silicon and sulphur derivatives R-M(=X)XH(M = C, Si; X = O, S; R = H, F, Cl, OH, NH2 and CH3)

Ab initio molecular orbital methods at the CBS-Q level of theory have been used to study the effect of substituent (F, Cl, NH2, OH and CH3) on the gas-phase acidities of formic acid, HCOOH, its silicon and sulphur derivatives R-M(= X)XH(M = C., Si; X = 0, S; R = F, Cl, OH, NH2 and CH3). For formic acid and its thio and dithio derivatives the acidity changes upon substitution are irregular and depend on both the type of substituent, position and degree of replacement of oxygen atoms by sulphur atoms. For sila carboxylic acids and their thio and dithio derivatives the calculated acidities regularly increase in the order: R-SiOOH < R-Si(=S)OH ≪ R-Si(=O)SH < R-SiSSH(R = H, F, Cl, OH, NH2 and CH3). The chloro derivatives are the strongest among the acids studied. The highest gas phase acidity (1277.6 kJmol−1) has been calculated for ClC(=S)OH.

Polarization of the excited states of twisted ethylene in a non-symmetrical environment

The polarization behavior of the low lying excited states in the vicinity of the perpendicularly twisted (D-2d) ethylene has been investigated in a quantum mechanical CISD approach, in which the quantum system was embedded in a polarized dielectric continuum modeling a non-symmetrical distribution of the solvent around the solute. The results show a strong polarization in the two lowest lying excited states in the region where the two vacuum energy surfaces of those states intersect, which strongly suggests that the lowering of the symmetry of the solvent shell can provide the adiabatic coupling for the avoided crossing between both potential energy surfaces. All examined values of epsilon, with the exception of the smallest investigated value(epsilon = 2.0), showed the polarization to keep intact on progressive twisting towards the perpendicular geometry.