F. Rogemond

Density functional theory study of some structural and energetic properties of small lithium clusters

Some properties of small Lin clusters (n up to 20) are theoretically investigated, within the density functional theory formalism. The structural properties are examined at the so‐called local level of approximation. For very small clusters (n≤8), the Lin conformations which are well known from ab initio calculations are found at very low computational cost. For n>8, optimal starting geometries are generated from two growth patterns, based on the increase of the number of pentagonal subunits in the clusters by adsorption of one or two Li atoms. Several new stable structures are proposed, for which the corresponding vibrational analysis is performed for n up to 18. The study of energetic properties and stability requires the use of gradient‐approximated functionals. Such functionals are used for the determination of the relative stability of these clusters. For example, we show that the icosahedral structure is the most favorable geometry for Li13, whereas this is not the case for Na13. Ionization potentia...

Analysis and assignment of the optical absorption transitions in CuCl2 with Gaussian density functional calculations

Some vibrational and electronic properties of the CuCl2 molecule are analyzed in order to characterize the electronic states and assign the bands seen on the optical absorption spectra. Equilibrium distances, total energies, electronic structure and harmonic vibrational frequencies of the low-lying states are calculated with the functional density method, in the unrestricted spin formalism, at two levels of theory for the exchange and correlation potential, the local and so-called non-local spin density approximations. All the results are compared with recent experimental data. The ground state is confirmed to be 2Πg, with a linear coordination as for the excited d→d states (2Σ+g, 2Δg) and charge-transfer states (2Πu, 2Σ+u). These low-lying states are in the following sequence: 2Σ+g, 2Πu, 2Δg, 2Σ+u, in order of increasing energies, different from those of previous Hartree-Fock results. Their harmonic vibrational frequencies are given and the oscillator strengths of the absorption transition from the ground state to these states are calculated. Therefore we propose a new assignment for the optical near infrared transitions (absorption and emission) to Σ+g↔2Πg, which would explain the lack of spin—orbit components of the emitting level in the experimental spectra, while the red ones could be 2Πu↔2Πg as already proposed in earlier works.

Computation of oscillator strengths from Kohn-Sham wave functions: the example of small lithium clusters

SummaryDensity functional calculations of ground and excited states of Lin (n≤8) clusters have been performed, within two different approaches. Using a set of Kohn-Sham orbitals to construct wave functions, the calculation of the oscillator strengths of the electric dipole transitions is performed. Our results have been tested at two levels: first the necessary comparison with the experimental data, second the comparison with high level CI (MRD-CI) calculations. This last point is not a trivial challenge, because such anab initio method leads for small clusters to a highly accurate description of the electronic structure and optical absorption spectra. Finally, this is also a new test for the capability of using Kohn-Sham orbitals to construct physically meaningful wave functions.Transition energies, oscillator strengths and finally optical absorption spectra presented here are in general in reasonable agreement with the experimental data and the MRD-CI calculations. That is very promising for bigger systems, with regard to the modest computational effort (CPU time and memory size) of density functional calculations.

Gradient-corrected Exchange Potential Functional with the correct asymptotic behaviour

Abstract Among the various conditions which should be satisfied by any exchange-correlation functional, some classes of conditions are less important for some practical applications to molecular systems. In this article, emphasis is brought to the 1 2 r and 1 r asymptotic behaviour which should be satisfied by energy functionals and potential functionals. It is shown that the rigorous derivation of the potential from the energy functional is not a critical requirement, and that, as far as preliminary results are concerned, the present potential functionals having the correct asymptotic behavior do not necessarily improve the theoretical determination of geometries of molecules. Some trends are deduced from calculations of molecular geometries with potential functionals possessing the correct asymptotic behaviour, and some hypotheses about missing features in the potential, like an improved shell structure are discussed.

Electronic structure calculations Bi4Ge3O12 with the MSXα method. Comparison with experimental data

Abstract MSXα calculations are performed on BiO 6 9− , GeO 4 4− and GeO 6 8− clusters in order to describe the electronic structure of the valence band of the scintillator Bi 4 Ge 3 O 12 . From the energetic diagrams, we obtain theoretical photoionization spectra of Bi 2 O 3 ,GeO 2 and Bi 4 Ge 3 O 12 which are compared to the experimental ESCA spectra. The wavefunctions of particular orbitals relative to the BiO 6 9− cluster, which contains the luminescent centers of Bi 4 Ge 3 O 12 , are plotted in several planes, allowing us to describe the BiBi and BiO bonds in the compound. The assignment of the near UV excitation transitions is performed and found consistent with the experimental data from optical spectroscopy.

DFT investigations of structural and energetical features of [H2O2]n (n=−1,0,1) potential energy surfaces

Oxywater (H2OO) is a proposed structural isomer of hydrogen peroxide (HOOH) that may serve as a transient intermediate in oxidation reactions. This molecule has been described by Pople et al. and Meredith et al. at ab initio level (HF and post HF). We have performed DF calculations at the local level of approximation and nonlocal level through various gradient‐corrected functionals. In addition the geometry has been optimized also by using a large polarized basis set in correlated (MP2 and CCSD(T) levels) ab initio MO calculations. Comparisons are made and this isomers seems to be a severe test for the accuracy of these functionals. Extensions towards the modelisation of the anionic and cationic structures, the interconversion and dissociation pathways are discussed and compared to the experimental data. Attention has been devoted to the monoanion analogues of hydrogen peroxide which equilibrium geometry is predicted to be almost planar with one hydrogen atom bridging both oxygen atoms and the unpaired el...

Theoretical calculations of the relative stability of Li6 isomers

The DFT approach has been used to study the relative stability of Li6 clusters. The three stable geometries, C2ν , C5ν , C3ν , have been obtained at the LSD level of approximation. For these geometries, we have also calculated the energies with different “non local” functionals and evaluate absorption spectra. Most of the results indicate the C2 ν , structure as the more stable.