An effective fragment model is developed to treat solvent effects on chemical properties andreactions. The solvent, which might consist of discrete water molecules, protein, or othermaterial, is treated explicitly using a model potential that incorporates electrostatics,polarization, and exchange repulsion effects. The solute, which one can most generally envision as including some number of solvent molecules as well, is treated in a fully ab initio manner, using an appropriate level of electronic structure theory. In addition to the fragment model itself, formulae are presented that permit the determination of analytic energy gradients and, therefore, numerically determined energy second derivatives (hessians) for the complete system. Initial tests of the model for the water dimer and water-formamide are in good agreement with fully abinitio calculations. An effective fragment model is developed to treat solvent effects on chemical properties and reactions. The solvent, which might consist of discrete water molecules, protein, or other material, is treated explicitly using a model potential that incorporates electrostatics, polarization, and exchange repulsion effects. The solute, which one can most generally envision as including some number of solvent molecules as well, is treated in a fully ab initio manner, using an appropriate level of electronic structure theory. In addition to the fragment model itself, formulae are presented that permit the determination of analytic energy gradients and, therefore, numerically determined energy second derivatives ͑hessians͒ for the complete system. Initial tests of the model for the water dimer and water-formamide are in good agreement with fully ab initio calculations.

Relationships between structures and properties (energy gaps, vertical ionization potentials (IP v ), vertical electron affinities (EA v ), and ligand binding energies) in small capped CdSe/CdTe nanoparticles (NPs) are poorly understood. We have performed the first systematic density functional theory (DFT) (B3LYP/Lanl2dz) study of the structural (geometries and ligand binding energies) and electronic (HOMO/LUMO energy gaps, IPs v , and EAs v ) properties of Cd n Se n /Cd n Te n NPs (n = 6, 9), both bare and capped with NH 3 , SCH 3 , and OPH 3 ligands. NH 3 and OPH 3 ligands cause HOMO/LUMO energy destabilization in capped NPs, more pronounced for the LUMOs than for the HOMOs. Orbital destabilization drastically reduces both the IP v and EA v of the NPs compared with the bare species. For SCH 3 -capped Cd 6 X 6 NPs, formation of expanded structures was found to be preferable to crystal-like structures. SCH 3 groups cause destabilization of the HOMOs of the capped NPs and stabilization of their LUMOs, which indicates a reduction of the IP v of the capped NPs compared with the bare species. For the Cd 9 X 9 NPs, similar trends in stabilization/destabilization of frontier orbitals were observed in comparison with the capped Cd 6 X 6 species. Also, pinning of the HOMO energies was observed for the NH 3 -and SCH 3 -capped NPs as a function of a NP size.

Simulated annealing methods have been used with the effective fragment potential to locate the lowest energy structures for the water clusters (H2O)n with n=6, 8, 10, 12, 14, 16, 18, and 20. The most successful method uses a local minimization on each Monte Carlo step. The effective fragment potential method yielded interaction energies in excellent agreement with those calculated at the ab initio Hartree-Fock level and was quite successful at predicting the same energy ordering as the higher-level perturbation theory and coupled cluster methods. Analysis of the molecular interaction energies in terms of its electrostatic,polarization, and exchange-repulsion/charge-transfer components reveals that the electrostatic contribution is the dominant term in determining the energy ordering of the minima on the (H2O)n potential energy surfaces, but that differences in the polarization and repulsion components can be important in some cases. 8, 10, 12, 14, 16, 18, and 20. The most successful method uses a local minimization on each Monte Carlo step. The effective fragment potential method yielded interaction energies in excellent agreement with those calculated at the ab initio Hartree-Fock level and was quite successful at predicting the same energy ordering as the higher-level perturbation theory and coupled cluster methods. Analysis of the molecular interaction energies in terms of its electrostatic, polarization, and exchange-repulsion/charge-transfer components reveals that the electrostatic contribution is the dominant term in determining the energy ordering of the minima on the (H 2 O) n potential energy surfaces, but that differences in the polarization and repulsion components can be important in some cases.

We report a comprehensive time-dependent density functional theory (TDDFT) study of one-photon and two-photon absorption (OPA and TPA, respectively) spectra for donor-pi-acceptor molecules. The calculated excitation energies were generally shown to be in good agreement with experiment, particularly when compared to results from measurements carried out in a nonpolar solvent, although the oscillator strengths were overestimated in some cases. Calculated TPA cross sections applying the two-state approximation were shown to be highly dependent on the form of the line-shape function used. Although a good agreement with experimental TPA spectra was generally observed, the wide range in the experimentally measured values and lack of systematic experimental data on solvent effects limited a detailed comparison as yet.