ORCID Profile
0000-0002-4372-6990
Current Organisation
Argonne National Laboratory
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Publisher: Elsevier BV
Date: 12-1989
Publisher: AIP Publishing
Date: 12-10-2006
DOI: 10.1063/1.2348881
Abstract: In an attempt to improve on our earlier W3 theory [A. D. Boese et al., J. Chem. Phys. 120, 4129 (2004)] we consider such refinements as more accurate estimates for the contribution of connected quadruple excitations (T̂4), inclusion of connected quintuple excitations (T̂5), diagonal Born-Oppenheimer corrections (DBOC), and improved basis set extrapolation procedures. Revised experimental data for validation purposes were obtained from the latest version of the Active Thermochemical Tables thermochemical network. The recent CCSDT(Q) method offers a cost-effective way of estimating T̂4, but is insufficient by itself if the molecule exhibits some nondynamical correlation. The latter considerably slows down basis set convergence for T̂4, and anomalous basis set convergence in highly polar systems makes two-point extrapolation procedures unusable. However, we found that the CCSDTQ−CCSDT(Q) difference converges quite rapidly with the basis set, and that the formula 1.10[CCSDT(Q)∕cc-pVTZ+CCSDTQ∕cc-pVDZ-CCSDT(Q)∕cc-pVDZ] offers a very reliable as well as fairly cost-effective estimate of the basis set limit T̂4 contribution. The T̂5 contribution converges very rapidly with the basis set, and even a simple double-zeta basis set appears to be adequate. The largest T̂5 contribution found in the present work is on the order of 0.5kcal∕mol (for ozone). DBOCs are significant at the 0.1kcal∕mol level in hydride systems. Post-CCSD(T) contributions to the core-valence correlation energy are only significant at that level in systems with severe nondynamical correlation effects. Based on the accumulated experience, a new computational thermochemistry protocol for first- and second-row main-group systems, to be known as W4 theory, is proposed. Its computational cost is not insurmountably higher than that of the earlier W3 theory, while performance is markedly superior. Our W4 atomization energies for a number of key species are in excellent agreement (better than 0.1kcal∕mol on average, 95% confidence intervals narrower than 1kJ∕mol) with the latest experimental data obtained from Active Thermochemical Tables. Lower-cost variants are proposed: the sequence W1→W2.2→W3.2→W4lite→W4 is proposed as a converging hierarchy of computational thermochemistry methods. A simple a priori estimate for the importance of post-CCSD(T) correlation contributions (and hence a pessimistic estimate for the error in a W2-type calculation) is proposed.
Publisher: Royal Society of Chemistry (RSC)
Date: 2023
DOI: 10.1039/D3CP04244H
Publisher: AIP Publishing
Date: 16-04-2012
DOI: 10.1063/1.3701762
Abstract: Photoelectron velocity map imaging is combined with one- and two-photon ionization to study the near threshold photoionization of the 2-butyne molecule. In this region, the photoabsorption and photoionization cross sections display a very intense broad feature that is assigned to an ℓ = 4, πg shape resonance. The effect of this shape resonance on the vibrational branching ratios and photoelectron angular distributions is explored. Theoretical calculations of the photoionization cross section and photoelectron angular distributions are in good agreement with the experiments. The results for 2-butyne are compared with those of acetylene, propyne, and 1-butyne, none of which show such significant enhancements near threshold, and the differences are rationalized in terms of the symmetries and orbital angular momenta of the highest occupied orbitals and the corresponding shape resonances. Expectations for larger alkynes and alkynyl radicals are also discussed. A preliminary measurement of the ionization energy of the 2-butyne dimer is also presented.
Publisher: IOP Publishing
Date: 28-09-1986
DOI: 10.1088/0022-3700/19/18/013
Abstract: The photoionisation spectrum of atomic sulphur prepared by the reaction of H atoms with H 2 S, is presented from its ionisation threshold to 925 AA. Autoionisation peaks assigned to quasidiscrete levels . . . ( 2 D degrees )nd 3 S degrees , 3 P degrees are sharp, whereas those designated . . . ( 2 D degrees )nd 3 D degrees are broad. This latter feature parallels the discontinuity observed between first-row and heavier elements in the noble gases and halogens. There is evidence for severe perturbation in the . . , ( 2 D degrees )nd 3 P degrees series, which could conceivably be attributed to a proximate 3s3p 5 3 P degrees state.
Publisher: IOP Publishing
Date: 28-09-1986
Publisher: Elsevier BV
Date: 06-2007
No related grants have been discovered for Branko Ruscic.