Diffuse Basis Functions Research Articles

Theoretical studies of excited states of H 3O and of core-excited NH 3: Comparisons with equivalent core analysis of NH 3 core-excitation spectra and implications for the radiation chemistry of aqueous systems

The H 3O radical has been studied within the ab initio LCAO SCF MO model. A flexible basis set including diffuse basis functions at both O and H has been used in order to represent the excited states adequately. Calculated excitation energies are 1.87, 2.87–3.16, and 3.36–3.47 eV; the calculated ionization energy is 4.75 eV. These represent well the experimental values (good to ±0.3 eV) of 1.6, 2.9, and 3.5 eV for excitation, and 5.0 eV for ionization, deduced by equivalent core analysis of high energy electron impact energy loss studies of NH 3. Similar explicit calculations on the N-1s core-excited states of NH 3 have also been made to examine directly the equivalent core concept. “Excited states” (relative to the lowest bound core-excited state) at 1.72, 2.85–3.09, and 3.32 eV, and the “ionization energy” of 4.68 eV, agree well with experiment and support the equivalent core concept. The possible significance of these H 3O results in the radiation chemistry of aqueous media is discussed in view of the fact that the maximum in the absorption spectrum of the hydrated electron lies near that of H 3O.

An ab initio molecular orbital study of the S N2 reaction of H − with HF

Ab initio SCF MO calculations have been performed for the reactants, products and reaction complex in the title reaction. The influence of diffuse and polarization basis functions in determining the presence of a reaction barrier was investigated. No barrier to the forward or reverse reactions was found.

Theoretical study of curve crossing: ab initio calculations on the four lowest 1Σ+ states of LiF

The potential curves of the four lowest 1Σ+ states of LiF have been studied by ab initio configuration interaction methods, using a contracted Gaussian basis set of better than double-ζ quality, augmented with diffuse basis functions on the fluorine atom. The choice of orbitals and selection of configurations were carried out with the objective of obtaining a balanced treatment of the four states and of the different regions of the potential curves. ``Generalized valence-bond'' orbitals, obtained from ground-state MCSCF calculations, together with ``improved virtual orbitals,'' were found suitable for the construction of the configuration functions. The latter were energy selected from all single and double excitations relative to a set of 12 ``reference configurations,'' chosen to include all contributions found important for any of the four states at any internuclear distance. The expected avoided crossing between the lowest covalent and ionic structures is found at an internuclear distance of about 11 bohr, compared to the experimentally deduced value of 14 bohr, due to the considerable difficulty in obtaining a balanced description of the electron correlation of the separated species, particularly the F− ion. A set of diabatic potential curves was generated from the two lowest adiabatic curves by a novel procedure based on the Rittner model of the ionic state. A set of adjusted adiabatic potentials was obtained from the diabatic curves after shifting the ionic diabatic curve downward by 0.42 eV to correct for the error in the computed electron affinity of the fluorine atom, resulting in a shift of the avoided crossing to the correct internuclear distance of 14 bohr. The dipole moments of the four states were computed as a function of internuclear distance and were interpreted in terms of the electronic structure.