Claude Pouchan

Vibrational computations beyond the harmonic approximation: Performances of the B3LYP density functional for semirigid molecules

The performances of the B3LYP density functional in the computation of harmonic and anharmonic frequencies were tested using 14 standard basis sets of double and triple zeta quality for a set of semirigid molecules containing from 4 to 12 atoms. The quality of the results is assessed by comparison with the most reliable computations available in the literature. The study reveals that the relatively cheap 6‐31+G(d,p) basis set performs a very good job for harmonic frequency calculations and that B3LYP anharmonicities are in close agreement with the reference values irrespective of the basis set used. On these grounds “hybrid force fields” are proposed to achieve the best compromise between computer time and quality of the results.

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.

Hyperpolarizability of GaAs dimer is not negative

We present a systematic study of the static electric hyperpolarizability of Ga(2)As(2). The authors rely on finite-field high-level ab initio calculations with carefully optimized basis sets. Their best values for the mean and the anisotropy of the dipole polarizability are alpha=158.57 and Deltaalpha=130.33e(2)a(0) (2)E(h) (-1). For the hyperpolarizability we propose an estimate gamma=(155+/-15)x10(3)e(4)a(0) (4)E(h) (-3), which does not agree with the negative value predicted by Lan et al. [J. Chem. Phys. 124, 094302 (2006)]. Density functional theory based methods yield values close to those predicted by conventional ab initio methods. The (hyper)polarizability components are particularly enhanced along the direction defined by the Ga-Ga axis.

Molecular geometry and polarizability of small cadmium selenide clusters from all-electron ab initio and Density Functional Theory calculations

We have calculated molecular geometries and electric polarizabilities for small cadmium selenide clusters. Our calculations were performed with conventional ab initio and density functional theory methods and Gaussian-type basis sets especially designed for (CdSe)(n). We find that the dipole polarizability per atom converges rapidly to the bulk value.

Doping-enhanced hyperpolarizabilities of silicon clusters: A global ab initio and density functional theory study of Si10 (Li, Na, K)n (n = 1, 2) clusters

A global theoretical study of the (hyper)polarizabilities of alkali doped Si(10) is presented and discussed. First, a detailed picture about the low lying isomers of Si(10)Li, Si(10)Na, Si(10)K, Si(10)Li(2), Si(10)Na(2), and Si(10)K(2) has been obtained in a global manner. Then, the microscopic first (hyper)polarizabilities of the most stable configurations have been determined by means of ab initio methods of high predictive capability such as those based on the Møller-Plesset perturbation and coupled cluster theory, paying extra attention to the (hyper)polarizabilities of the open shell mono-doped systems Si(10)Li, Si(10)Na, Si(10)K, and the influence of spin contamination. These results were used to assess the performance of methods of low computational cost based on density functional theory (DFT) in the reliable computation of these properties in order to proceed with an in-depth study of their evolution as a function of the alkali metal, the cluster composition, and the cluster structure. The most interesting outcomes of the performed (hyper)polarizability study indicate that while alkali doping leaves the per atom polarizability practically unaffected, influences dramatically the hyperpolarizabilities of Si(10). The lowest energy structures of the mono-doped clusters are characterized by significantly enhanced hyperpolarizabilities as compared to the analogue neutral or charged bare silicon clusters Si(10) and Si(11), while, certain patterns governed by the type and the number of the doping agents are followed. The observed hyperpolarizability increase is found to be in close connection with specific cluster to alkali metal charge transfer excited states and to the cluster structures. Moreover, an interesting correlation between the anisotropy of the electron density, and the hyperpolarizabilities of these systems has been observed. Finally, it is important to note that the presented method assessment points out that among the various DFT functionals used in this work, (B3LYP, B3PW91, BhandHLYP, PBE0, CAM-B3LYP, LC-BLYP, LC-BPW91) only B3PW91 and PBE0 out of the seven provided a consistent quantitative performance for both polarizabilities and hyperpolarizabilities with respect to the ab initio methods utilized here. On the other hand, the long range corrected functionals LC-(U)BLYP and LC-(U)BPW91 (μ = 0.47) failed to supply quantitatively accurate hyperpolarizability results in all the studied clusters while the CAM-(U)B3LYP functional performs satisfactory only in the case of the Na and K doped systems.

Ab initio configuration interaction determination of the overtone vibrations of methyleneimine in the region 2800–3200 cm−1

Ab initio configuration interaction (CI) potential function calculated from a 6-311G** basis set at a MP2 level of theory is used to compute the vibrational energy levels of methyleneimine CH2NH between 2800 and 3200 cm−1. The most important configurations selected by an iterative process by means of a variational perturbational method are diagonalized. Results show a perfect agreement with the most reliable experimental values and predict the ν5+ν6, ν4+ν6, and ν4+ν5 unobserved combinations.

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.

Effect of the synthetic methodology on molecular architecture: from statistical to gradient copolymers

Styrene-butyl acrylate (S-BuA) copolymers were synthesized by nitroxide-mediated controlled radical polymerization using an alkoxyamine as an initiator. Using different synthetic methodologies, statistical copolymers were be obtained by batch nitroxide mediated polymerization while the gradient composition was a forced gradient by continuous addition of S during BuA polymerization (semi-batch process). These gradient copolymers have been studied by H NMR and size exclusion chromatography to characterize the gradient composition molecular structure. The evolution of the composition was correlated with the glass transition temperature () of the copolymers. The gradient copolymers exhibit one with a value in between the of polystyrene and poly(butyl acrylate), indicating that the materials did not present well defined microphase separation. Specific organization at the air-polymer interface of such copolymers has also been demonstrated by comparison between classical and attenuated total reflection (ATR) Fourier transform infra-red spectra. This bulk soft matter assembly was confirmed by AFM analysis, which showed a different morphology at the surface and in the bulk following removal of the top layer. Moreover, for the most well defined gradient composition, a specific nano-structuring was demonstrated by small angle neutron scattering. The preliminary rheological properties of these gradient copolymers were studied and are discussed in relation with their molecular structure.