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Showing papers by "Ernest R. Davidson published in 1985"


Journal ArticleDOI
TL;DR: In this paper, the authors calculated water-water two-body interaction potentials with an ab initio MO method by varying not only the OO distance but also the OH distance, and obtained ROO=2.79 A, ROH=0.977 A with the binding energy of 15.8 kcal/mol per H2O molecule.
Abstract: The OO distance in ice (2.76 A) is much shorter than in water dimer (2.98 A). No first principle potential function has successfully described the observed OO shrinkage. We have calculated water–water two‐body interaction potentials with an ab initio MO method by varying not only the OO distance but also the OH distance. New analytical fits of two‐body potential functions have been obtained. The nearest‐neighbor three‐body potential has been evaluated for proton‐ordered ice–Ih structures. With ab initio one, two, and nearest‐neighbor three‐body potentials, ∠HOH fixed at the monomer value, we have been able to obtain ROO=2.79 A, ROH=0.977 A with the binding energy of 15.8 kcal/mol per H2O molecule for proton‐ordered antiferroelectric ice Ih and ROO=2.85 A, ROH=0.972 A with the binding energy of 14.3 kcal/mol per molecule for ferroelectric ice Ih. The three‐body interaction, aided by the two‐body interaction, contributes to the OO shrinkage. Factors that would favor larger ROH stretch and ROO shrinkage have...

92 citations


Journal ArticleDOI
TL;DR: In this article, the authors reexamined two radicals whose ab initio hyperfine properties exhibited unusually large deviations from experiment in a recent study of such properties, and showed that incorporating vibrational averaging effects, MCSCF orbitals and large CI reference spaces resulted in a reduction of deviation from experiment to 14% for the formaldehyde radical cation.
Abstract: Two of the radicals whose ab initio hyperfine properties exhibited unusually large (30%) deviations from experiment in a recent study of such properties have been reexamined. Inclusion of vibrational averaging effects, MCSCF orbitals and large CI reference spaces resulted in a reduction of the deviation from experiment to 14% for the formaldehyde radical cation. New experimental data on the methoxy radical shows a much larger proton isotropic hyperfine value than previous experimental and theoretical studies. The best current calculations were unable to resolve the discrepancy. The ethyl radical was also included in this study since good experimental beta proton data is available for comparison.

77 citations


Journal ArticleDOI
TL;DR: It is shown that for cases where the transfer function is wavelength dependent and independent, superresolution can be attained for simulated objects within 1/30th of the Rayleigh distance.
Abstract: In this paper, we consider the problem of attaining superresolution in the case of two point objects which are within the Rayleigh distance of each other and possess differing spectral characteristics. Furthermore, we assume that the image data is a set of intensity values as a function of both position coordinates and wavelength coordinates. If linear superposition holds, well-known linear algebraic methods of rank and eigenanalysis can be used to estimate the number of spectrally distinct objects and, in favorable cases, to estimate the spatial distribution and spectrum of each. Computer simulations of this strategy show its efficacy in detecting, spatially resolving, and spectrally identifying two impulse objects. We show that for cases where the transfer function is wavelength dependent and independent, superresolution can be attained for simulated objects within 1/30th of the Rayleigh distance.

58 citations



Journal ArticleDOI
TL;DR: In this article, the effects of variations in basis set and choice of CI reference space configurations on electron affinities were investigated using balanced [8s, 5p, 4d, 2f, 1g] contracted Gaussian basis sets.
Abstract: In order to better understand the requirements for obtaining ab initio electron affinities which agree with experiment to within 0.1–0.2 eV, a systematic study has been made of the effects of variations in basis set and choice of CI reference space configurations. Using balanced [8s, 5p, 4d, 2f, 1g] contracted Gaussian basis sets it was possible to obtain values of 1.22 eV for carbon (experimental=1.27 eV) and 1.29 eV for oxygen (experimental=1.46 eV). Enlargement of the (s,p) portion of the basis to near Hartree–Fock limit quality resulted in substantially more correlation energy being recovered but no significant change in the computed electron affinities.

49 citations



Journal ArticleDOI
TL;DR: In this article, Ab initio CI calculations are reported on the lowest quintet, triplet, and singlet states of FeII(P) (NH3)2.
Abstract: Ab initio CI calculations are reported on the lowest quintet, triplet, and singlet states of FeII(P) (NH3)2. The lowest singlet state has strong mixing between the configuration (dxy2 (dπ)4 and (dxy)2(dπ)3egπ*. The lowest quintet is mixed between 6A1g)dπ and (6A1g)egπ*, where 6A1g refers to the high-spin ferric configuration. We calculate many low-energy states as 3(ππ*) ring and metal triplet and quintet configurations [“triptriplets” and “tripquintets”]. The calculations also show low-energy charge-transfer configurations of ring anion excited quartets and ferric quartets and sextets [“quartquartets” and “quartsextets”]. The farthest red x,y-polarized bands of the experimental spectra of low-spin hemoproteins are identified as dxy egπ* or dπ d mixed with dπ d and the z-polarized bands are assigned as dπ egπ*. The farthest red x,y-polarized bands of the high-spin hemoproteins are identified as excited quartsextet states. Picosecond transients observed in FeII(TPP) (pip)2 are attributed to an initial 1(dπ egπ*) state, which inter-system crosses to high-spin states that lose one ligand.

35 citations







Journal ArticleDOI
TL;DR: In this article, the relationship between the deuterium quadrupole coupling constants and the inverse cube of the bond length is refined and discussed, and extensive calculations at both the semi-empirical and ab initio levels are carried out to determine the electric field gradient at the hydrogen positions.
Abstract: Single crystals of α‐glycine with two, three, and five of the protons replaced by deuterium are studied at 4.2 K using electron–nuclear double resonance (ENDOR) spectroscopy to detect the NMR transitions of the distant deuterons on diamagnetic molecules. Complete deuteron quadrupole interaction tensors, and indirectly the electric field gradient tensors, are determined for all the labeled positions. Proton dipolar coupling tensors are also obtained for the methylene hydrogens in the case of glycine‐d3. The relationship between the deuterium quadrupole coupling constants and the inverse cube of the bond length is refined and discussed. The resulting structural information including X–D bond lengths and bond angles is compared with data obtained by neutron diffraction. In addition, extensive calculations at both the semiempirical and ab initio levels are carried out to determine the electric field gradient at the hydrogen positions. Both intramolecular and lattice contributions are considered.

Journal Article
TL;DR: In this article, RHF et CI indiquant une geometrie optimale avec deux atomes P pyramidalises, non equivalents electroniquement, non-equivalent electroniquements.
Abstract: Calculs ab initio RHF et CI indiquant une geometrie optimale avec deux atomes P pyramidalises, non equivalents electroniquement

Journal ArticleDOI
TL;DR: Les calculs MNDO sont utilises for estimer la geometrie de l'etat fondamental, les densites electroniques atomiques, la chaleur de formation, le potentiel d'ionisation, les valeurs de ΔH en phase gazeuse pour la protonation and les Valeurs ΔH et E a pour la trifluoroacetylation and la nitration electrophiles, avec respectivement l'anhydride triffluoroacetique protone and le nitrate de trif
Abstract: Les calculs MNDO sont utilises pour estimer la geometrie de l'etat fondamental, les densites electroniques atomiques, la chaleur de formation, le potentiel d'ionisation, les valeurs de ΔH en phase gazeuse pour la protonation et les valeurs ΔH et E a pour la trifluoroacetylation et la nitration electrophiles, avec respectivement l'anhydride trifluoroacetique protone et le nitrate de trifluoroacetyle, de l'azupyrene


Journal ArticleDOI
TL;DR: In this article, the lowest doublet, quartet, and sextet states of FeIII(P)(NH3)2 were calculated for the first time, and the low-spin ground state was calculated as (dxy2 (dπ)3 with dxy(dπ)-4 higher by 0.15 eV.
Abstract: Ab initio CI calculations are reported on the lowest doublet, quartet, and sextet states of [FeIII(P)(NH3)2]+. The low-spin ground state is calculated as (dxy2 (dπ)3 with dxy(dπ)4 higher by 0.15 eV. The near-ir bands at ∼1 eV observed in low-spin ferriheme proteins are attributed to (π dπ) transitions. The lowest high-spin state is 6A1g, and the near-ir transitions of the high-spin ferriheme proteins observed at ∼1.2 eV are attributed to higher 6[tripsextet] excited states [i.e., ring triplet, metal sextet]. The 30-ps “triplet” transient populated with low quantum yield observed in laser-flash studies on FeIII(TPP)CI [TPP = tetrapbenylporphyrin] may be an 1[tripsextet] state.