Didier Bégué

A comparison of two methods for selecting vibrational configuration interaction spaces on a heptatomic system: ethylene oxide.

Two recently developed methods for solving the molecular vibrational Schrodinger equation, namely, the parallel vibrational multiple window configuration interaction and the vibrational mean field configuration interaction, are presented and compared on the same potential energy surface of ethylene oxide, c-C(2)H(4)O. It is demonstrated on this heptatomic system with strong resonances that both approaches converge towards the same fundamental frequencies. This confirms their ability to tackle the vibrational problem of large molecules for which full configuration interaction calculations are not tractable.

Between geometry, stability, and polarizability: density functional theory studies of silicon clusters Sin (n = 3-10).

The relationship between the polarizability, stability, and the geometry of small-size silicon clusters has been investigated by the density functional theory methods. Results obtained at local density approximation/Vosko-Wilk-Nusair and general gradient approximation/BLYP levels with polarized even-tempered basis set of quadruple zeta quality are presented and compared with those obtained by the B3LYP method, as well as with the ab initio results in recent literature. We have found that the polarizability is directly related to the size of the energy gap between symmetry-compatible bonding and antibonding molecular orbitals, but not necessarily to the size of the HOMO-LUMO (highest occupied molecular orbital-lowest unoccupied molecular orbital) gap. Furthermore, we have defined two structural parameters, namely, the averaged Si-Si distances and the standard deviation of the Si-Si distances, which were found to correlate remarkably well with the binding energy of the clusters and the HOMO-LUMO gap, respectively. These straightforward correlations would, therefore, provide a means to predict the physical properties, in particular, the polarizability and the stability, simply based on the structural information of the cluster.

Accurate dipole polarizabilities of small silicon clusters from ab initio and density functional theory calculations

We report conventional ab initio and density functional theory calculations for the static dipole polarizability for the small silicon clusters Si3 to Si7. Our effort relies on the design of flexible basis sets of Gaussian-type functions. The dependence of the calculated mean polarizability per atom (ᾱ/n) on basis set type and level of theory is brought forth and discussed. The effect of electron correlation is found to be small for all studied clusters. The density functional theory based methods are seen to predict reliable values for (ᾱ/n). By introducing and analyzing the differential mean polarizability per atom, ᾱdiff/n=ᾱ(Sin)/n−ᾱ(Si), we show that in fact ab initio and density functional theory calculations yield distinctly different pictures of the polarizability of small silicon clusters. Computational strategies are proposed for the extension of theoretical studies to larger structures.

Nitrile Imines: Matrix Isolation, IR Spectra, Structures, and Rearrangement to Carbodiimides

The structures and reactivities of nitrile imines are subjects of continuing debate. Several nitrile imines were generated photochemically or thermally and investigated by IR spectroscopy in Ar matrices at cryogenic temperatures (Ph-CNN-H 6, Ph-CNN-CH(3)17, Ph-CNN-SiMe(3)23, Ph-CNN-Ph 29, Ph(3)C-CNN-CPh(3)34, and the boryl-CNN-boryl derivative 39). The effect of substituents on the structures and IR absorptions of nitrile imines was investigated computationally at the B3LYP/6-31G* level. IR spectra were analyzed in terms of calculated anharmonic vibrational spectra and were generally in very good agreement with the calculated spectra. Infrared spectra were found to reflect the structures of nitrile imines accurately. Nitrile imines with IR absorptions above 2200 cm(-1) have essentially propargylic structures, possessing a CN triple bond (typically PhCNNSiMe(3)23, PhCNNPh 29, and boryl-CNN-boryl 39). Nitrile imines with IR absorptions below ca. 2200 cm(-1) are more likely to be allenic (e.g., HCNNH 1, PhCNNH 6, HCNNPh 43, PhCNNCH(3)17, and Ph(3)C-CNN-CPh(3)34). All nitrile imines isomerize to the corresponding carbodiimides both thermally and photochemically. Monosubstituted carbodiimides isomerize thermally to the corresponding cyanamides (e.g., Ph-N═C═N-H 5 → Ph-NH-CN 8), which are therefore the thermal end products for nitrile imines of the types RCNNH and HCNNR. This tautomerization is reversible under flash vacuum thermolysis conditions.

New parallel software (P_Anhar) for anharmonic vibrational calculations: Application to (CH3Li)2

We present the development of a new parallel computer code (P_Anhar_v1.0) to calculate the vibrational spectrum of medium size molecules using a variational algorithm. The method is applied to the determination of a complete quartic anharmonic force field (B3LYP/cc‐pVTZ) for methyllithium, leading to a new interpretation of experimental data.

Calculation of IR frequencies and intensities in electrical and mechanical anharmonicity approximations: Application to small water clusters

We present a method for automatic computation of infrared (IR) intensities using parallel variational multiple window configuration interaction wave functions (P_VMWCI(2) algorithm). Inclusion of both mechanical and electrical anharmonic effects permits fundamental vibrational frequencies, including combinations and overtones, to be assigned. We use these developments to interpret the near-IR (NIR) and mid-IR (MIR) spectra of individual water clusters (H2O)(n) (n=1-4). Cyclic and linear systems are studied to provide equivalent reference theoretical data to investigate the structure of water as a function of density using NIR and MIR experimental spectra. Various density functional theory methods for generating the potential energy surface have been compared to reference results obtained at the CCSD(T) level [X. Huang et al., J. Chem. Phys. 128, 034312 (2008)]. For cyclic clusters, the IR intensities and frequencies obtained using B1LYP/cc-pVTZ are found to be in very good agreement with the available experimental values and of the same orders of magnitude as the reference theoretical values. These data are completed by the vibrational study of linear systems.

Linear and nonlinear optical properties of a series of Ni-dithiolene derivatives.

Some linear and nonlinear optical (NLO) properties of Ni(SCH)(4) and several of its derivatives have been computed by employing a series of basis sets and a hierarchy of methods (e.g., HF, DFT, coupled cluster, and multiconfigurational techniques). The electronic structure of Ni(SCH)(4) has been also analyzed by using CASSCF/CASPT2, ab initio valence bond, and DFT methods. In particular we discuss how the diradicaloid character (DC) of Ni(SCH)(4) significantly affects its NLO properties. The quasidegeneracy of the two lowest-energy singlet states 1 (1)A(g) and 1 (1)B(1u), the clear DC nature of the former, and the very large number of low-lying states enhance the NLO properties values. These particular features are used to interpret the NLO properties of Ni(SCH)(4). The DC of the considered derivatives has been estimated and correlated with the NLO properties. CASVB computations have shown that the structures with Ni(II) are the dominant ones, while those with Ni(0) and Ni(IV) have negligible weight. The weights of the four diradical structures were discussed in connection with the weight of the structures, where all the electrons are paired. Comparative discussion of the properties of Ni(SCH)(4) with those of tetrathia fulvalene demonstrates the very large effect of Ni on the properties of the Ni-dithiolene derivatives. A similar remarkable effect on the NLO properties is produced by one or two methyl or C(3)S groups. The considered Ni-dithiolene derivatives have exceptionally large NLO properties. This feature in connection with their other physical properties makes them ideal candidates for photonic applications.

Ab initio finite field (hyper)polarizability computations on stoichiometric gallium arsenide clusters GanAsn (n=2–9)

We report reliable ab initio finite field (hyper)polarizability values at Hartree-Fock and second order Moller-Plesset perturbation theory (MP2) levels of theory for different geometrical configurations of small gallium arsenide clusters Ga(n)As(n) with n=2-5. We relied on all-electron basis sets and pseudopotentials suitable for (hyper)polarizability calculations. In each case, we used structures that have been established in the literature after we optimized their geometries at B3LYP/cc-pVTZ-PP level of theory. Our results suggest that the first order hyperpolarizability (beta) is much more sensitive to the special geometric features than the second order hyperpolarizability (gamma). For the most stable configurations up to ten atoms the second order hyperpolarizability at MP2 level of theory varies between 15 x 10(4) and 32 x 10(4) e(4)a0 (4)Eh(-3). In addition, we examined the polarizability per atom evolution versus the cluster size for Ga(n)As(n) with n=2-9. Our work extends earlier theoretical studies which were limited to eight atoms and exposes that the polarizability/atom of the most stable stoichiometric configurations up to Ga(9)As(9) continues the monotonic downward trend with increasing size. Lastly, from the methodological point of view, our analysis shows that apart from polarizabilities, augmented pseudopotentials yield reliable first and second hyperpolarizability values as well.

Modern mass spectrometry for studying mass-independent fractionation of heavy stable isotopes in environmental and biological sciences

This is the first review to focus specifically on the application of modern mass spectrometry techniques for studying mass-independent or anomalous isotope fractionation for ‘heavy’ elements. This review covers a period of ten years, starting from 2000. In the first part of the manuscript, we address theoretical aspects of mass-independent isotope fractionation, such as nuclear volume and nuclear spin effects. Secondly, most published articles on this topic to date are reviewed. Mass-independent isotope fractionation was determined by ICP-MS, MC-ICP-MS or TIMS for nineteen elements so far: Hg, Sn, Cd, Zn, Te, Sr, Pb, Cr, U, Ti, Ni, Mo, Ru, Ba, Nd, Sm, Gd, Yb and Hf, as reported in about fifty publications. In addition, mass-independent fractionation was observed for some other elements, silicon, germanium and sulfur, by other techniques.

Electrostatic interaction schemes for evaluating the polarizability of silicon clusters.

Electrostatic interaction schemes have been applied to predict the evolution of the polarizability in Si(n) clusters of increasing size (n=3-19). Both on-site polarization and charge transfer effects have been included in the interaction scheme, of which the values have been compared to B3LYP/6-311G(*) and other first principles results. To reproduce the pattern of the variation of the B3LYP average polarizability per Si atom as a function of the cluster size, the atomic polarizability employed in the interaction scheme should amount to roughly 80% of the bulk atomic polarizability. However, this results in a systematic underestimation of the polarizability per Si atom by about 25%, whereas increasing the atomic polarizability value leads to excessive variations of the polarizability per Si with the cluster size. An improved agreement is obtained when incorporating a charge transfer contribution, at least for sufficiently large clusters, substantiating the fact that in large clusters electrostatic effects are dominant over quantum effects. This charge transfer atomic polarizability term has been modeled by a simple function, which evolves linearly with the difference of Cartesian coordinates between the atom and the center of mass and that has been verified using B3LYP/6-311G(*) calculations. In the case of the prediction of the polarizability anisotropy, a similar atomic polarizability corresponding to 80% of the bulk atomic polarizability has been shown suitable to reproduce the B3LYP results, whereas inclusion of charge transfer effects can slightly improve the agreement, provided the amount of charge transfer increases with the size of the cluster.