Lattice Distortions Around a Tl+ Impurity in NaI:Tl+ and CsI:Tl+ Scintillators. An Ab Initio Study Involving Large Active Clusters

Ab initio Perturbed Ion cluster-in-the-lattice calculations of the impurity centers NaI:Tl+ and CsI:Tl+ are pressented. We study several active clusters of increasing complexity and show that the lattice relaxation around the Tl+ impurity implies the concerted movement of several shells of neighbors. The results also reveal the importance of considering a set of ions that can respond to the geometrical displacements of the inner shells by adapting selfconsistently their wave functions. Comparison with other calculations involving comparatively small active clusters serves to assert the significance of our conclusions. Contact with experiment is made by calculating absorption energies. These are in excellent agreement with the experimental data for the most realistic active clusters considered.Comment: 7 pages plus 6 postscript figures, LaTeX. Submmited to Phys, Rev.

Ab Initio Calculation of the Lattice Distortions induced by Substitutional Ag- and Cu- Impurities in Alkali Halide Crystals

An ab initio study of the doping of alkali halide crystals (AX: A = Li, Na, K, Rb; X = F, Cl, Br, I) by ns2 anions (Ag- and Cu-) is presented. Large active clusters with 179 ions embedded in the surrounding crystalline lattice are considered in order to describe properly the lattice relaxation induced by the introduction of substitutional impurities. In all the cases considered, the lattice distortions imply the concerted movement of several shells of neighbors. The shell displacements are smaller for the smaller anion Cu-, as expected. The study of the family of rock-salt alkali halides (excepting CsF) allows us to extract trends that might be useful at a predictive level in the study of other impurity systems. Those trends are presented and discussed in terms of simple geometric arguments.Comment: LaTeX file. 8 pages, 3 EPS pictures. New version contains calculations of the energy of formation of the defects with model clusters of different size

DFT exchange: sharing perspectives on the workhorse of quantum chemistry and materials science

In this paper, the history, present status, and future of density-functional theory (DFT) is informally reviewed and discussed by 70 workers in the field, including molecular scientists, materials scientists, method developers and practitioners. The format of the paper is that of a roundtable discussion, in which the participants express and exchange views on DFT in the form of 302 individual contributions, formulated as responses to a preset list of 26 questions. Supported by a bibliography of 777 entries, the paper represents a broad snapshot of DFT, anno 2022

LES CHENILLES PROCESSIONNAIRES EN FRANCE, LA LUTTE CONTRE LA CHENILLE PROCESSIONNAIRE DU PIN EN FRANCE

Les chenilles processionnaires (Lépidoptères Thaumétopoeîdés) sont représentées, en France, par 3 espèces : Thaumetopoea pityocampa, Thaumetopoea pinivora et Thaumetopoea processionea. Après une description des différentes espèces, leurs cycles de développement sont étudiés : on distingue une processionnaire d'hiver et des processionnaires d'été. La répartition des espèces est inégale et aucune n'occupe l'ensemble du territoire français. Cette répartition conditionne l'importance des dégâts occasionnés par ces chenilles défoliatrices. Elles ont aussi un pouvoir pathogène toxique sur l'homme et les animaux, qui s'enveniment par contact avec les poils urticants. Le ravageur des résineux le plus important en France est Thaumetopoea pityocampa, la processionnaire du pin. De ce fait des méthodes de lutte ont été développées : méthodes mécaniques, chimiques (diflubenzuron) ou biologiques (Bacilles thuringiensis). Cette lutte est organisée et comprend des systèmes de surveillance précédant des traitements bien réglementés.MAISONS-ALFORT-Ecole Vétérin (940462302) / SudocSudocFranceF

Obtaining a gradient-corrected kinetic-energy functional from the Perdew-Wang exchange functional

Lee, Lee, and Parr (LLP) have shown that the kinetic energy can be written in the same form as Becke's exchange energy. This conjecture of LLP has been generalized to another exchange functional, namely, the Perdew-Wang exchange functional. As demonstrated by Lee and Parr, the exchange energy can be written K=πFFsΓ(r,s)drds with Γ(r,s)=||γ(r,s)||2¯/n2(r), where ||γ(r,s)||2¯ is the spherical average of ||γ(r,s)||2. Using the generalization of LLP's conjecture, it is demonstrated that Γ(r,s)= e-s2/β(r)+a[s4/β02(r)]e-s2/β0(r), a=const, β0(r)=5[3π2n(r)]-2/3. At zeroth order, β(r)=β0(r), the function Γ(r,s), gives exactly the modified Gaussian approximation proposed by Lee and Parr

Gradient-corrected exchange potential with the correct asymptotic behavior and the corresponding exchange-energy functional obtained from the virial theorem

In density-functional theory (DFT), Perdew, Parr, Levy, and Balduz [Phys. Rev. Lett. 49, 1691 (1982)] have shown that for all the electronic systems, the energy of the highest occupied molecular orbital (HOMO) is equal to the negative of the ionization potential. This equality is not recovered within the different approximations of the exchange-correlation functional proposed in the literature. The main reason is that the exchange-correlation potentials of various functionals used in DFT calculations decay rapidly to zero whereas they should exhibit a Coulombic asymptotic -1/r behavior. In this work we propose a gradient-corrected (GC) exchange potential with a correct asymptotic -1/r form for large values of r. The energy of the HOMO calculated with this potential is improved compared to the local-density approximation (LDA) and to the GC functionals widely used in the DFT. Our HOMO eigenvalues are compared to the optimized-potential-model eigenvalues which are the exact values for the exchange-only potential. Using the fact that the LDA satisfies the virial theorem, the exchange energy corresponding to this GC exchange potential can be calculated under a simple assumption