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A computer program has been written to perform analytic derivative calculations of magnetic response properties based on uncontracted multireference configuration interaction (MRCI) wavefunctions. At this point, orbital relaxation is neglected in this treatment and the basis functions are assumed to be independent of the perturbation. It is therefore equivalent to an untruncated sum-over-states (SOS) approach to the g-tensor. Hence, the treatment is more rigorous than all previous MRCI g-tensor calculations that have used truncated SOS formulations. In the present calculations, MRCI calculations were performed on top of full valence complete-active space self-consistent field (CASSCF) reference wavefunctions without truncation of the reference or correlation spaces. Results are reported for two types of spin-orbit coupling operators, a simple effective nuclear charge approximation and the spin-orbit mean field (SOMF) approximation. The effects of orbital relaxation are discussed. The values obtained can be used in future calibration studies. [ABSTRACT FROM AUTHOR]

Coupled cluster and density-functional theory calculations of the electronic circular dichroism (ECD) spectra and optical rotatory dispersion (ORD) of (S)-methyloxirane and (S)-methylthiirane have been carried out for comparison to gas-phase experimental data. Coupled cluster simulations of the ECD spectra of both molecules are in excellent agreement with experiment, both for the positions of the transitions and for the signs and magnitudes of the rotational strengths. Density-functional theory yields the correct structure of the ECD spectra, but the transition energies are shifted to lower energies by up to 0.3 eV. Density-functional ORD curves of (S)-methylthiirane exhibit incorrect dispersion due to an early onset of the Cotton pole. The coupled cluster ORD curves of (S)-methylthiirane give the correct sign of the rotations, but overestimate their magnitude by up to 50% at shorter wavelengths. Although temperature-dependent harmonic vibrational corrections improve the comparison between coupled-cluster theory and experiment at 633 nm, such corrections produce an incorrect sign of the rotation at 355 nm. [ABSTRACT FROM AUTHOR]

The article profiles scientist Péter Pulay. Pulay was born in Veszprém, Hungary on September 20 1941. His graduate work was on the electronic structure of silane. He has a vast interest in vibrational spectra and molecular force constants. Furthermore, one of his most important contributions in the field was an efficient method for calculating the chemical shift in nuclear magnetic resonance.

Empirical valence bond (EVB) surfaces have been constructed for 2-pyridone-2-hydroxypyridine proton transfer and for the Claisen rearrangement of allyl vinyl ether at the MP2/6-311 + G(d,p) level of theory. A distributed Gaussian approach is used to approximate the interaction matrix elements. Parameters for the distributed Gaussians are determined by fitting to energy, gradient and Hessian data obtained from ab initio electronic structure calculations at one to nine points along the reaction path. An efficient DIIS (direct inversion of iterative subspace) method is used to solve the fitting equations. Criteria for choosing internal coordinates for the representation of the potential energy surfaces and for the interaction matrix element are discussed. Practical techniques for determining the placement and exponents of the Gaussians are described. With one set of s-, p- and d-type Gaussians at the transition state, the error in the energy along the reaction path is less than 10 kJ mol-1 for pyridone tautomerization. Five sets of Gaussians reduces the error to less than 5 kJ mol-1 and seven Gaussians drops the error below 1 kJ mol-1. The Claisen rearrangement is more challenging and requires seven Gaussians to achieve an error of less than 4 kJ mol-1 for energies along the reaction path. [ABSTRACT FROM AUTHOR]

The optimized geometries, total energies, and vibrational frequencies of the polyacenes and perfluoropolyacenes in their closed-shell singlet and lowest triplet states have been studied using the B3LYP method in conjunction with double-ζ plus polarization (DZP) basis sets. The equilibrium structures of all the polyacenes and perfluoropolyacenes are planar, with D2h symmetry, and the singlet and triplet geometries display interesting patterns for each family of molecules. The largest singlet-triplet structural changes appear in the outermost C-C distances. The small perfluoropolyacene systems have ground state singlet states, and the singlet-triplet energy gaps decrease as the number of linearly fused benzene rings increases. With the number of rings greater than seven, the triplet state of the perfluorinated species falls below the closed-shell singlet. For the parent polyacenes, the same shift occurs between n = 8 and n = 9. Correspondingly, the LUMO-HOMO gap for the singlet state decreases with increasing number of linearly fused perfluoro benzene rings. The predicted singlet-triplet separations for the perfluorinated compounds range from 54 kcal mol-1 (perfluoronaphthalene) to -7 kcal mol-1 (n = 10). [ABSTRACT FROM AUTHOR]

The adsorption of small organic molecules onto vicinal Au-stabilized Si(111) surfaces is shown to be a versatile route towards controlled growth of ordered organic-metal hybrid one-dimensional nanostructures. Density functional theory is used to investigate the site-specific adsorption of toluene-3,4-dithiol (TDT) molecules onto the clean Si(553)-Au surface and onto a co-doped surface whose steps are passivated by hydrogen. We find that the most reactive sites involve bonding to silicon at the step edge or on the terraces, while gold sites are relatively unfavored. H passivation and TDT adsorption both induce a controlled charge redistribution within the surface layer, causing the surface metallicity, electronic structure, and chemical reactivity of individual adsorption sites to be substantially altered. [ABSTRACT FROM AUTHOR]