Proceedings - Interdisciplinary Center for Nanotoxicity

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140 Conference on Current Trends in Computational Chemistry 2009 Empirical methods for Electronically Metastable Anions Becky Weber and Thomas Sommerfeld Southeastern Louisiana University, Department of Chemistry and Physics, SLU 10878 Hammond, LA 70402 USA Most closed‐shell molecules have a negative vertical attachment energy. In other words, when an electron is attached to a closed‐shell molecule at its equilibrium geometry, as a rule, the energy goes up, and the radial anion that is formed is a short‐lived metastable species, an anion that is unstable with respect to decaying back into a neutral molecule and a free electron. In contrast to electronically stable states, metastable states, also referred to as resonances, have finite lifetimes, and are embedded into a continuum of neutral‐plus‐free‐ electron states. Owing to their continuum nature these states cannot be described by square‐ integrable wavefunctions, but for an ab initio treatment one needs to use explicit, or at least implicit, scattering approaches. Examples for explicit scattering methods are the R‐matrix and the Complex‐Kohn method, examples for implicit scattering methods are the complex scaling, the absorbing potential, and the stabilization methods. Needless to say that both explicit and implicit scattering methods are computationally far more expensive than bound‐state calculations for systems of similar size, and these approaches are thus limited to fairly small molecules. π* LUMO of furan. This orbital is only well‐defined using a minimal basis set. It can still be computed with larger basis sets, however, only if the basis set contains no diffuse functions [3]. When basis sets with diffuse functions are used the π* orbital will mix with orbitals describing the neutral and a free electron, and in the complete basis set limit the π* orbital will completely “dissolve” in the neutral‐plus‐free‐electron continuum. Due to the large computational effort associated with ab initio treatments of metastable anions several empirical methods have been developed to compute the energy, though not the lifetime, of certain classes of metastable radical anion. The most economical method is to empirical scale the LUMO energy [1,2]. For this type of approach it is crucial to use a pure valence basis sets without any diffuse function as the use of diffuse functions gives LUMOs with too much continuum character [1,2,3], and for a set of hydrocarbons Dunning's double‐zeta set (without any polarization or diffuse functions) turned out to yield the best correlation between the experimentally determined attachment energy and the computed LUMO energy. More recently it has been shown that for a larger class of compounds density functional calculations for the lowest‐energy attachment state can also yield energies close to experiment [4,5]. Again, the basis set, or artificial confinement, play the role of an empirical parameter, as calculations at the basis set limit will yield attachment energies of 0 (for long‐range corrected functions) or negative infinity (for self‐interaction uncorrected functional such as B3LYP). Here we investigate the performance of empirical schemes for classes of metastable states not in their fitting (or basis set selection) procedure, consider alternative empirical schemes that do not rely on scaling the correlation‐energy contributions to the attachment
Conference on Current Trends in Computational Chemistry 2009 energy, and compare the results of empirical schemes with ab initio stabilization results. The following figure shows results obtained from the empirical correlation scheme developed described in [1]. This approach was developed exclusively for hydrocarbons (blue squares) , but as our calculations show, it also works for some compounds with CO ‐systems (red triangles) and CN ‐systems (yellow triangles). However, it does not work for *‐ anions (the green diamond showing no good correlation is chloroform), and, notably, it does not work for the *‐ state of CO2 (red triangle at 4.3 eV, 3.6 eV). 1. S. W. Staley, J. T. Strnad, J. Phys. Chem. 98, 116 (1994) 2. D. Chen, G. A. Gallup, J. Chem. Phys. 93, 8893 (1990) 3. M. Guerra, Chem. Phys. Lett. 167, 315 (1990) and Chem. Phys. Lett. 197, 205 (1992) 4. M. Puiatti, D. Mariano, A. Vera, A. B. Pierini, Phys. Chem. Chem. Phys. 10, 1394, (2008) 5. N.Sablon, F. De Proft, P. Geerlings, D. J. Tozer, Phys. Chem. Chem. Phys. 9, 5880, (2007) 141
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Organizing Committee Shonda Allen-H
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68 Ag 36 (D 3d) Ag 42 (D 3) Ag 48 (
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76 Table 2. Calculated scale factor
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86 Isodesmic Model Conference on Cu
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Proceedings - Interdisciplinary Center for Nanotoxicity

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