. Can. J. Chem. 70, 6 12 (1992). Relativistic compact effective potentials (RCEP), which replace the atomic core electrons in molecular calculations, have been derived from numerical Dirac-Fock atomic wavefunctions using shape-consistent valence pseudo-orbitals and an optimizing procedure based on an energy-overlap functional. Potentials are presented for the third-, fourth-, and fifthrow atoms of the Periodic Table (excluding the lanthanide series). The efficiency of molecular calculations is enhanced by using compact Gaussian expansions (no more than three terms) to represent the radial components of the potentials, and energy-optimized, shared-exponent, contracted-Gaussian atomic orbital basis sets. Transferability of the potentials has been tested by comparing calculated atomic excitation energies and ionization potentials with values obtained from numerical relativistic Hartree-Fock calculations. For the alkali and alkaline earth atoms, core polarization potentials (CPP) have been derived which may be added to the RCEP to make possible accurate molecular calculations without explicitly including core-valence correlating configurations in the wavefunction. Introduction two rows (Li-Ar) of the Periodic Table ( 5 ) . We designatedThe use of atomic effective core potentials (ECP) and model potentials (MP) to eliminate chemically inactive atomic core electrons from quantum mechanical calculations has become routine in the past decade. The development of such potentials in the early 1970's and applications through the mid-1980's have been reviewed previously (1). The use of such potentials in molecular calculations has gained widespread acceptance, and sets of effective potentials are included in widely distributed quantum chemistry programs such as HONDO ( 2 ) , GAMESS (3), and GAUSSIAN (4). We previously published accurate compact effective potentials (CEP) and matching basis sets for the atoms of the first '~u t h o r to whom correspondence may be addressed. 'NRC-NAS Postdoctoral Fellow, NIST, 1984NIST, -1986. Current address: California State University, San Marcos, CA 92096, U.S.A.Primed in Canada -the potentials "compact" because they are represented analytically by small Gaussian expansions and, therefore, offer significant economy in molecular calculations where the computer time required to construct the electronic integrals is proportional to the complexity of the potentials. In this report, we present effective potentials and basis sets of similar quality for the third, fourth, and fifth rows of the Periodic Table derived from numerical, relativistic, Dirac-Fock atomic wavefunctions. The analytic expansions of these POtentials are also limited to a few Gaussian terms (three or less), so we have designated them as "relativistic compact effective potentials" (RCEP).Recently, several compilations of model potentials and effective core potentials for use in molecular calculations have appeared in the literature. For the heavier atoms, relativistic effects have been incorporated by deriving the potentials ...

Compact effective potentials, which replace the atomic core electrons in molecular calculations, are presented for atoms in the first and second rows of the periodic table. The angular-dependent components of these potentials are represented by compact one- and two-term Gaussian expansions obtained directly from the appropriate eigenvalue equation. Energy-optimized Gaussian basis set expansions of the atomic pseudo-orbitals, which have a common set of exponents (shared exponents) for the s and p orbitals, are also presented. The potentials and basis sets have been used to calculate the equilibrium structures and spectroscopic properties of several molecules. The results compare extremely favorably with corresponding all-electron calculations.

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.

Metal/molecule/metal transport junctions can transport charge in the elastic scattering (Landauer) regime if the injection gap is large and the molecule is relatively short. Stochastic switching and broad conduction peak distributions have been observed in such junctions. We examine the effect of altering interface geometry on transport, using density functional calculations. For most structures, variations in conductance of order 0-300% are found, but when an atomic wire of Au binds to the molecule, symmetry changes can modify currents by a factor of 10(3).

The hybrid density functional (DFT) method B3LYP was used to study the mechanism of the methane hydroxylation reaction catalyzed by a non-heme diiron enzyme, methane monooxygenase (MMO). The key reactive compound Q of MMO was modeled by (NH 2 )(H 2 O)Fe(µ-O) 2 (η 2 -HCOO) 2 Fe(NH 2 )(H 2 O), I. The reaction is shown to take place via a bound-radical mechanism and an intricate change of the electronic structure of the Fe core is associated with the reaction process. Starting with I, which has a diamond-core structure with two Fe IV atoms, L 4 Fe IV (µ-O) 2 Fe IV L 4 , the reaction with methane goes over the rate-determining H-abstraction transition state III to reach a bound-radical intermediate IV, L 4 Fe IV (µ-O)(µ-OH(‚‚‚CH 3 ))Fe III L 4 , which has a bridged hydroxyl ligand interacting weakly with a methyl radical and is in an Fe III -Fe IV mixed valence state. This short-lived intermediate IV easily rearranges intramolecularly through a low barrier at transition state V for addition of the methyl radical to the hydroxyl ligand to give the methanol complex VI, L 4 Fe III (OHCH 3 )-(µ-O)Fe III L 4 , which has an Fe III -Fe III core. The barrier of the rate-determining step, methane H-abstraction, was calculated to be 19 kcal/mol. The overall CH 4 oxidation reaction to form the methanol complex, I + CH 4 f VI, was found to be exothermic by 39 kcal/mol.

Comparison of the successive ionization potentials in planar, nonaromatic hydrides with those in the corresponding perfluoro compounds demonstrates that MO's are stabilized 2.5-4 eV by the substitution, whereas the stabilization can be an order of magnitude smaller for MO's. This preferential stabilization of MO's is termed "the perfluoro effect." The generality of the perfluoro effect was demonstrated experimentally and theoretically using the ethylene-tetrafluoroethylene, water-oxygen difluoride, formaldehyde-carbonyl fluoride, and diimide-difluorodiazine pairs. He(I) and He(II) photoelectron spectra of all of these molecules except diimide are presented, together with those on the acetone-hexafluoroacetone, azomethane-hexafluoroazomethane, and butadiene-1,1,4,4-tetrafluorobutadiene pairs. In the pairs containing the methyl and trifluoromethyl groups, and MO's are approximately equally stabilized by the fluorine atoms, showing that the trifluoromethyl group effectively destroys thedistinction in these molecules. Gaussian orbital calculations of double-^quality were performed for the smaller molecular pairs; the Koopmans' theorem values are in good agreement with experiment. Analysis of the wave functions shows that in the perfluoro compounds, the MO's are appreciably delocalized over the fluorine atoms, and are strongly stabilized by the high effective nuclear charge of that atom. In the r MO's, the delocalization onto the fluorine atoms is much less, and its stabilizing effect is counteracted by a strong antibond between the fluorine atom and the atom to which it is bonded.

The high-resolution He i and He ii photoelectron spectra of all fluoromethanes in the series CH4 to CF4 and their deuterated analogs have been recorded and are compared with the Koopmans' theorem results of near-Hartree–Fock calculations performed in a Gaussian basis. The agreement is very good in general and offers an unambiguous assignment of almost all of the bands observed. In particular, repeated correlations are demonstrated between the compositions of the orbitals from which the electrons are ejected and the characters of the resulting photoelectron bands. Identifiable trends throughout the series are stressed and an anomalous feature in the CF4 spectrum is noted. Jahn–Teller effects in CH4 and CH3F are clearly evident, but as expected, they are not observed in CHF3 and CF4. Comparison of the photoelectron spectra excited with He i and He ii radiation shows wide variations in the relative intensities of various bands in certain of the more symmetric molecules, suggesting that relative intensities can be a poor measure of relative orbital degeneracies. Mass-spectrometric appearance-potential data are briefly discussed in the light of the photoelectron results. The carbon and fluorine 1s binding energies as measured with 1254-eV x rays are shown to be electronically adiabatic. The accurate determination of the lower ionization potentials of these molecules leads readily to the assignment of several of their electronic transitions as lower members of ns and np Rydberg series.

The first four bands in the gas-phase spectra of amides, carboxylic acids, and acyl fluorides are thought to be n → π*, n → 3s Rydberg, π → π*, and n → 3p Rydberg excitations. That the second and fourth bands are Rydberg, whereas the first and third are valence shell is demonstrated in a comparison of gas-phase and condensed-phase absorption and circular dichroism spectra. All-electron, SCF Gaussian orbital calculations are also presented which qualitatively explain the trends in the spectra of HCOX molecules, and predict several quantities of interest, such as upper-state dipole moments and magnetic transition moments, which have not been measured as yet.