Jean-Pierre Flament

A two-step uncontracted determinantal effective Hamiltonian-based SO–CI method

We present a new two-step uncontracted spin-orbit configuration interaction (CI) method which automatically accounts for spin-orbit polarization effects on multiconfigurational wave functions by selecting the single excitations having a significant spin-orbit interaction with a chosen determinantal reference space. This approach is in the line of a conventional two-step method, as a sophisticated correlation treatment in a scalar relativistic approximation is carried out in the first step. In the second step, we define a model space which includes a set of reference configurations able to represent all the wanted states along with singly excited configurations selected with the spin-orbit (SO) operator. We then exploit the first-step calculation in order to include correlation effects via an effective Hamiltonian technique and diagonalize the full matrix on the determinantal basis. The method combines the advantages of both one-step and conventional two-step SO–CI methods; it intends to treat efficiently ...

An Investigation of the Accuracy of Different DFT Functionals on the Water Exchange Reaction in Hydrated Uranyl(VI) in the Ground State and the First Excited State

We discuss the accuracy of density functional theory (DFT) in the gas phase for the water-exchange reactions in the uranyl(VI) aqua ion taking place both in the electronic ground state and in the first excited state (the luminescent (3)Δg state). The geometries of the reactant and intermediates have been optimized using DFT and the B3LYP functional, with a restricted closed-shell formalism for the electronic ground state and either an unrestricted open-shell formalism or the time-dependent DFT method for the (3)Δg state. The relative energies have been computed with wave-function-based methods such as Møller-Plesset second-order perturbation theory, or a minimal multireference perturbative calculation (minimal CASPT2); coupled-cluster method (CCSD(T)); DFT with B3LYP, BLYP, and BHLYP correlation and exchange functionals; and the hybrid DFT-multireference configuration interaction method. The results obtained with second-order perturbative methods are in excellent agreement with those obtained with the CCSD(T) method. However, DFT methods overestimate the energies of low coordination numbers, yielding to too high and too low reaction energies for the associative and dissociative reactions, respectively. Part of the errors appears to be associated with the amount of Hartree-Fock exchange used in the functional; for the dissociative intermediate in the ground state, the pure DFT functionals underestimate the reaction energy by 20 kJ/mol relative to wave-function-based methods, and when the amount of HF exchange is increased to 20% (B3LYP) and to 50% (BHLYP), the error is decreased to 13 and 4 kJ/mol, respectively.

Quantum Chemical and Molecular Dynamics Study of the Coordination of Th(IV) in Aqueous Solvent

In this work, we investigate the solvation of tetravalent thorium Th(IV) in aqueous solution using classical molecular dynamics simulations at the 10 ns scale and based on polarizable force-field approaches, which treat explicitly the covalent character of the metal-water interaction (and its inherent cooperative character). We have carried out a thorough analysis of the accuracy of the ab initio data that we used to adjust the force-field parameters. In particular, we show that large atomic basis sets combined with wave function-based methods (such as the MP2 level) have to be preferred to density functional theory when investigating Th(IV)/water aggregates in gas phase. The information extracted from trajectories in solution shows a well-structured Th(IV) first hydration shell formed of 8.25 ± 0.2 water molecules and located at about 2.45 ± 0.02 Å and a second shell of 17.5 ± 0.5 water molecules at about 4.75 Å. Concerning the first hydration sphere, our results correspond to the lower bounds of experimental estimates (which range from 8 to 12.7); however, they are in very good agreement with the average of existing experimental data, 2.45 ± 0.02 Å. All our results demonstrate the predictable character of the proposed approach, as well as the need of accounting explicitly for the cooperative character of charge-transfer phenomena affecting the Th(IV)/water interaction to build up reliable and accurate force-field approaches devoted to such studies.

A theoretical study of nonadditive effects in four water tetramers

The properties of four water tetramers have been investigated using ab initio computations at the MP2/6-311+G(2d,2p) level. The four tetramers correspond to various molecular arrangements. The computations showed that, with respect to the water dimer, a significant reinforcement of the hydrogen bonds occurs due to cooperative effects when water molecules adopt a pattern where they are simultaneously donor and acceptor of a hydrogen bond. This is consistent with an interpretation of hydrogen bonding in terms of charge-transfer phenomena in the hydrogen bond region and suggests that the origin of a part of the cooperativity occurring in water aggregates arises from cooperative charge-transfer phenomena. Analysis of the intermolecular interaction energies, carried out by both the ab initio supermolecule approach and the SIBFA molecular mechanics procedure, showed the inductive effects to be mainly responsible for the tetramer nonadditive effect.

An ab initio theoretical study of the electronic structure of UO2+ and [UO2(CO3)3]5-

The electronic spectra up to 50,000 cm(-1) of uranyl(V) both as a bare ion, UO2(+), and coordinated with three carbonate ligands, [UO2(CO3)3]5-, are presented. Solvent effects were treated by a nonequilibrium continuum solvent model. The transition energies were obtained at the spin-orbit level using relativistic wave function based multiconfigurational methods such as the complete active space self-consistent field method (CASSCF)and the complete active space with second-order perturbation theory (CASPT2) followed by a calculation of the spin-orbit effects at the variation-perturbation level. Earlier relativistic intermediate Hamiltonian Fock space coupled-cluster calculations on the spectrum of the bare uranyl(V) ion were extended to investigate the influence of electron correlation effects on spacings between the electronic states. This study is an attempt to contribute to an enhanced understanding of the electronic structure of actinyl ions. Both spectra show transitions within nonbonding orbitals and between nonbonding and antibonding orbitals as well as charge transfers from the uranyl oxygens to uranium. The ground state in UO2(+) is found to be 2Φ(5/2u), corresponding to the σ(u)(2)φ(u)(1) configuration, while in [UO2(CO3)3]5-, it is 2Δ(3/2u), arising from the σ(u)(2)δ(u)(1) configuration. It is remarkable that the excited state corresponding to an excitation from the nonbonding δ(u) to the uranyl antibonding 3π(u)* molecular orbital is significantly lower in energy in the carbonate complex, 6623 cm(-1), than that in the bare ion, 17,908 cm(-1). The first ligand (carbonate) to metal charge-transfer excitation is estimated to occur above 50,000 cm(-1). The reported results compare favorably with experiment when available.

Ab initio study of the acetylene and vinylidene dications fragmentation

The C2H++2 fragmentation processes have been studied using the complete active space self‐consistent field method followed by a multireference perturbative configuration interaction, in order to interpret recent charge separation spectroscopy experiments. For two‐body processes, the calculated appearance thresholds of the C2H+/H+ and CH+/CH+ fragment pairs are in good agreement with the experimental data. It is shown that the C2H++2→CH++CH+ dissociation occurs with an important rotation of the CH+ ions. The presence of the CH+2 ion is explained by a preliminary isomerization of acetylene to vinylidene dication. This reaction has been studied for the lowest lying states of C2H++2 (3Σ−g and 1Δg) and compared with other acetylenic ions isomerizations (C2H2, C2H+2, C2H−2). For three‐body processes, the calculations are consistent with the mechanisms proposed by the experimentalists.

The core excitation of pyridine and pyridazine: An electron spectroscopy and ab initio study

The carbon and nitrogen K-shell excitation spectra of gaseous pyridine and pyridazine were recorded using the electron-energy loss spectroscopy under electric-dipole conditions (2 keV, small angle) with a resolution of 0.2 eV. Ab initio Configuration interaction calculations in the frame of the equivalent core model were performed in order to help in the assignment of the spectral features. The spectra are dominated by the transitions to the 1π* and σ* type orbitals. The C1s spectra of both molecules are close to that of benzene: The intensity of Rydberg transitions are enhanced by an important valence σC–H* character; the 1s→3π* transition is mixed with double excitations and give rise to several states, some of them lying above the ionization thresholds. Finally, the N1s spectra of both molecules are similar to the s-triazine one.

Investigation of the vibrational properties of CN– on a Pt electrode by in situ VIS–IR sum frequency generation and functional density calculations

The vibrational properties of CN– pseudohalide ions adsorbed on a Pt(110) single-crystal electrode in aqueous neutral solution are studied using in situ VIS–IR sum frequency generation (SFG) and compared to theoretical results obtained from density functional theory (DFT) calculations on PtNC– and PtCN– molecular ions. As observed previously on a polycrystalline electrode, the adsorption behaviour of both ions depends drastically on the electrode potential and on the immersion potential in a CN–-containing solution. When the electrode is immersed at negative potential, two absorption bands are detected in the spectral range of the CN– stretching vibration modes on a surface site. The first resonance at 2070 cm–1 is already observed at –1.4 V (Ag/AgCl), in the hydrogen evolution potential region, with an initial potential tuning rate of 20 cm–1. A second narrow resonance, peaking at 2150 cm–1, starts growing at –0.6 V (Ag/AgCl). The occurrence of two resonances, corresponding to N-bound and C-bound CN– species, is clearly evidenced for a single-crystal electrode, and appears as a genuine property of the system, not a side effect induced by surface defects. The polycrystalline structure and the defects influence the shape, the position and the width of the resonances, not the basic spectroscopic properties of the system. This result is further supported by new density functional calculations of the vibrational properties of the PtCN– and PtNC– molecular ions.

Ab initio embedded cluster study of the excitation spectrum and Stokes shifts of Bi3+-doped Y2O3.

The ab initio embedded cluster method coupled with correlated spin-orbit calculations has been used to interpret the excitation spectrum of a Bi(3+)-doped yttria crystal. Our results indicate that the Bi(3+) impurity can absorb light over a wider energy range in the C(2) site than in the S(6) site. Even if the computed absorption energies seem to be about 0.4 eV too high with respect to the experimental peaks for both sites, it is noteworthy that the embedded cluster model renders 93% of the large crystal redshift, about 6 eV. The determination of the geometry relaxation of the first shell of oxygen neighbors upon electronic excitation shows that the Stokes shift is smaller in the S(6) site than in the C(2) site. Combining all these results confirms the assignment of the violet emission to the S(6) site and that of the green emission to the C(2) site, as proposed by Boulon [J. Phys. (Paris) 32, 333 (1971)]. In addition, the nature of the metastable states which lie below the emitting ones and are responsible for the temperature dependence of the fluorescence lifetimes is discussed.

Improvement of the ab initio embedded cluster method for luminescence properties of doped materials by taking into account impurity induced distortions: The example of Y2O3:Bi3+

New ab initio embedded-cluster calculations devoted to simulating the electronic spectroscopy of Bi(3+) impurities in Y(2)O(3) sesquioxide for substitutions in either S(6) or C(2) cationic sites have been carried out taking special care of the quality of the environment. A considerable quantitative improvement with respect to previous studies [F. Real et al. J. Chem. Phys. 125, 174709 (2006); F. Real et al. J. Chem. Phys. 127, 104705 (2007)] is brought by using environments of the impurities obtained via supercell techniques that allow the whole (pseudo) crystal to relax (WCR geometries) instead of environments obtained from local relaxation of the first coordination shell only (FSR geometries) within the embedded cluster approach, as was done previously. In particular the uniform 0.4 eV discrepancy of absorption energies found previously with FSR environments disappears completely when the new WCR environments of the impurities are employed. Moreover emission energies and hence Stokes shifts are in much better agreement with experiment. These decisive improvements are mainly due to a lowering of the local point-group symmetry (S(6)-->C(3) and C(2)-->C(1)) when relaxing the geometry of the emitting (lowest) triplet state. This symmetry lowering was not observed in FSR embedded cluster relaxations because the crystal field of the embedding frozen at the genuine pure crystal positions seems to be a more important driving force than the interactions within the cluster, thus constraining the overall symmetry of the system. Variations of the doping rate are found to have negligible influence on the spectra. In conclusion, the use of WCR environments may be crucial to render the structural distortions occurring in a doped crystal and it may help to significantly improve the embedded-cluster methodology to reach the quantitative accuracy necessary to interpret and predict luminescence properties of doped materials of this type.