Showing papers by "Ernest R. Davidson published in 1996"

Refinement of the Asymptotic Z Expansion for the Ground-State Correlation Energies of Atomic Ions

Abstract: Davidson and co-workers (Phys. Rev. A 1991, 44, 7071; 1993, 47, 3649) have estimated the nonrelativistic correlation energies and relativistic corrections to ionization potentials for atomic ions with up to 18 electrons. However, due to the lack of theoretical values for the high-Z limits and lack of more accurate compilations of experimental ionization potentials, the analysis for 11−18 electrons required further investigation. In this work, we have accurately determined the exact high-Z limit employing degenerate second order perturbation theory for the correlation contribution to the energies of atomic ions with 3−18 electrons. We have also incorporated the experimental compilation of the electron affinity data of Hotop and Lineberger for the low-Z limit. This high-Z limit is compared with the results of the LYP correlation energy functional. The LYP correlation functional is also compared with the correlation energy of electrons in an external harmonic potential of infinite force constant.

The Spatial Extent of the V State of Ethylene and Its Relation to Dynamic Correlation in the Cope Rearrangement

Abstract: The classical problem of the extent of Rydberg−valence mixing for the V state of ethylene is re-examined using a large basis set and several different approaches to selecting configurations. The results are interpreted in terms of the general need for dynamic σ-π correlation in π-electron systems. The effect of dynamic correlation on the activation energy of the Cope rearrangement is especially noted.

An electron spin resonance investigation of vanadium dioxide ( 51 V 16 O 2 and 51 V 17 O 2 ) and 51 V 17 O in neon matrices with preliminary assignments for VO 3 and V +2 : Comparison with ab initio theoretical calculations

Abstract: The first spectroscopic characterization of the VO2 radical is reported along with new results for V17O and tentative assignments for the VO3 and V+2 radicals. These vanadium radicals were investigated in neon matrices at 4 K by electron spin resonance utilizing conventional high temperature vaporization and pulsed laser ablation generation methods. A detailed ESR study of VO2 showed it to be nonlinear with a 2A1 ground state; the g tensor analysis reveals the presence of an excited electronic state (2B1) approximately 1 eV above the ground state. This excited state prediction and the observed nuclear hyperfine interactions (A tensors) for 51V and 17O were compared with theoretical results obtained from various ab initio computational methods. Ab initio calculations with an extended basis set were performed at various levels of theory including UHF, ROHF, CAS‐SCF, and MR‐SDCI (multireference single and double configuration interaction). While UHF calculations of the hyperfine interaction were grossly in e...

Electron spin resonance matrix isolation and ab initio theoretical investigations of 69,71gah2, 69,71gad2, h69,71gach3, and d69,71gacd3

Abstract: First time electron spin resonance studies are reported for various isotopomers of GaH2 and HGaCH3. The radicals were generated in neon matrices at 4 K by the ultraviolet photoexcitation of Ga which undergoes insertion reactions with H2 and CH4. Ab initio calculations with a large uncontracted basis and configuration interaction with all single excitations from the spin‐restricted Hartree–Fock configuration gave good agreement with the experimental results and supported the free atom comparison method interpretation of the hyperfine interactions. A comparison with similar radicals is presented, including BH2, AlH2, HAlCH3, HAlOH, and AlH+.

Orbital momentum profiles and binding energy spectra for the complete valence shell of molecular fluorine

Abstract: The first electronic structural study of the complete valence shell binding energy spectrum of molecular fluorine, encompassing both the outer and inner valence regions, is reported. These binding energy spectra as well as the individual orbital momentum profiles have been measured using an energy dispersive multichannel electron momentum spectrometer at a total energy of 1500 eV, with an energy resolution of 1.5 eV and a momentum resolution of 0.1 a.u. The measured binding energy spectra in the energy range of 14–60 eV are compared with the results of ADC(4) many-body Green's function and also direct-CI and MRSD-CI calculations. The experimental orbital electron momentum profiles are compared with SCF theoretical profiles calculated using the target Hartree-Fock approximation with a range of basis sets and with Density Functional Theory predictions in the target Kohn-Sham approximation with non-local potentials. The truncated (aug-cc-pv5z) Dunning basis sets were used for the Density Functional Theory calculations which also include some treatment of correlation via the exchange and correlation potentials. Comparisons are also made with the full ion-neutral overlap amplitude calculated with MRSD-CI wavefunctions. Large, saturated basis sets (199-GTO) were employed for both the high level SCF near Hartree-Fock limit and MRSD-CI calculations to investigate the effects of electron correlation and relaxation.

Electron momentum spectroscopy experiments and calculations for the production of excited states of He+ and

Abstract: The (e,2e) cross-section for transitions to the n = 2 final state of He+ and the 2sσg, final state of have been measured, relative to the cross-section for the transitions to the respective ground state ions, using a highly sensitive momentum dispersive multichannel electron momentum spectrometer. The experimental results for He are compared with plane wave impulse approximation (PWIA) cross-section calculations carried out using two previously published GI wavefunctions and also with two cross-section calculations based on explicitly correlated wavefunctions with energy errors of less than 10 nHartree. The H2 results are compared with calculations by J.W. Liu and V.H. Smith Jr. (Phys. Rev. A, 31, 3003 (1985); erratum: Phys. Rev. A, 39, 3703 (1989)). For both He and H2, significant differences are observed between the measured relative cross-sections and those calculated using the PWIA. While the measurements for He differ from previous work, the results for H2 are consistent with some earlier measurements.

Insights into theoretical quantum chemistry from electron momentum spectroscopy

Abstract: Most basis sets used in quantum chemistry are designed to get the correct charge and momentum density in the region important for covalent bonding. The (e,2e) cross section measured by electron momentum spectroscopy (EMS) emphasizes the low-momentum, large r, region of the wave function. Improving the description of this part of the wave function for water has resulted in good agreement with (e,2e) data. Because the hydrogen bond is sensitive to the long-range tail of the wave function, this has simultaneously led to an improved description of the hydrogen bond in the water dimer. The satellite region of the binding energy spectrum gives information about the excited states of the cation that is not available at present from any other form of spectroscopy. Calculations seeking agreement with the binding-energy spectra and the momentum distribution associated with satellite peaks have led to the most complete catalog of the cation excited states for ethylene. Here we report the assignment of the excited st...

Ab initio calculations on excited molecular ions of ethylene and acetylene

Abstract: Ab initio calculations of the energy and wave functions for neutral molecules and excited molecular ions are used to interpret the EMS and PES spectra of acetylene and ethylene. The five satellite regions of acetylene are assigned to intensity borrowed from 2σg, 3σg and 2σu primary hole states by ion states with configurations (πu)−2 (δg)1, (πu)−2 (3σg)−1 (πg)2, and (πu)−1 (3σg)−1 (πg)1. The Dyson orbitals for the satellite regions of acetylene and ethylene are shown to differ from the Dyson orbitals of the primary holes. The importance of choosing basis sets appropriate to the excited states of the ion is illustrated.