Modeling the Effects of Solvation on the Structure and Properties of Optical Limiting Materials Using Ab Initio Quantum
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In the Effective Fragment Potential (EFP) method, discrete solvent molecules interact with the solute, which is treated fully ab initio, while the solvent molecules interact with the solute through three types of semiclassical terms: distributed multipolar expansions, induced dipoles at distributed polarizable points, and exchange repulsion. While the multipoles and polarizabilities needed for the first two terms can be easily obtained from an ab initio calculation on the solvent molecule of interest, obtaining the exchange repulsion, which is a purely quantum effect, is more difficult. An accurate parameterization of this term for water has been developed 4 by calculating the water dimer interaction ab initio at a large number of geometries and fitting the results to a gaussian functional form. The resulting EFP for water (which is included in the GAMESS program) has been found to accurately simulate water solvent molecules on formamide, glycine, and glutamic acid.' However, this approach is laborious, and thus does not lend itself to the efficient production of EFPs for a wide variety of solvents. A more efficient method for obtaining reasonably accurate exchange repulsion potentials has been developed by Jensen and Gordon.6 This method approximates the exchange potential in terms of overlap matrix elements and Fock matrix elements of the solvent, expressed in the localized molecular orbital (LMO) basis. The main disadvantage of this method is that only exchange repulsion effects are included, while the fit used previously could be designed to include components of the interaction energy that are not explicitly part of the EFP, such as charge transfer and higher-order coupling. However, this new approach has a stronger theoretical basis, and it is expected that eventually the other parts of the interaction energy will be included in the EFP in a rigorous and efficient manner. 307
Mat. Res. Soc. Symp. Proc. Vol. 479 01997 Materials Research Society
Using the new method for the exchange repulsion, EFPs have been obtained for methanol and chloroform. The accuracy of these EFPs has been validated by comparing to full ab initio calculations on the dimer of each solvent molecule and on formamide with three solvent molecules. The system of formamide with three EFP water molecules has been tested against full ab initio calculations, and the EFP model has been shown to accurately mimic the interaction energy and to reproduce the effects on solute geometry. The performance of these two new EFPs will be compared to that of the water EFP. Ab initio calculations have been carried out on several substituted porphyrins. In this paper, we report the results of EFP calculations with the solvents water, methanol, and chloroform on zinc tetraphenyl-octobromyl-porphyrin (ZnTPOBrP). This porphyrin has a saddle-like structure, and apparently it retains this saddle-like structure upon solvation. METHODS The GAMESS
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program has been used to carry out all calculations. The DZP
(double-zeta with polarization) basis of Dunning and H
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