A variational density matrix approach with nonlocal effective potential

We show that using the Colle-Salvetti correlation-energy functional [TCA 37, 329 (1975)] in the Hartree-Fock-type procedure suggested by Kohn and Sham [PR 140, A1133 (1965) Sec.IIB.Nonlocal Effective potential], one can calculate quite accurately electronic properties of systems in which the "dynamical" correlation energy is dominant. We compare our results with those obtained by Grabo and Gross [CPL 240, 141 (1995)] using the optimized effective potential method, and we discuss characteristics and advantages of our procedure

A general algorithm for fitting efficiently triple differential cross sections of atomic double photoionization

We propose an effective procedure to fit triple differential cross sections of atomic double photoionization processes, which is based on a general expression of the transition amplitude between arbitrary states of the target atom and the parent ion, with the transition operator expressed at any order of its multipolar expansion. The major advantage of our expression, that in the dipole approximation is equivalent to those of Manakov (1996 J. Phys. B: At. Mol. Opt. Phys. 29 2711) and Malegat (1997 J. Phys. B: At. Mol. Opt. Phys. 30 251), is that it is expressed only in terms of elementary angular functions (Clebsch\u2013Gordan coefficients, spherical harmonics and 6 − j factors). Therefore our expression can be easily implemented in a general code for any kinematic condition and any order of the multipolar expansion of the transition operator. Our fitting procedure takes into account also the finite instrumental resolution in measuring energies and angles. Test calculations on helium and argon show that this further capability is often essential to remove important discrepancies between simulated and measured angular distributions

Structural evolution and counterion dynamics in Emeraldine Salt by Car-Parrinello molecular dynamics.

By means of Car Parrinello Molecular Dynamics (CP-MD), we have studied the dynamics of the polymer chains and Cl- counterions of the crystalline Emeraldine Salt (ES-II) obtained by acidification of Emeraldine Base II (EB-II) with HCl. After heating the system, we have followed its dynamics at T=270 K for about 7 Ps, showing the evolution from an initial Pc2a space-group lattice configuration with Cl- ions in bipolaronic positions, to a final P21221 lattice configuration with Cl- ions in polaronic positions. This transition has been characterized by an initial large displacement of the chlorine counterions followed by the structural rearrangements of the polymer chains in the route to the P21221 space-group lattice

Base and salt 3D forms of Emeraldine polymers EB-II and ES-II by Car-Parrinello Molecular Dynamics.

We have studied the structural properties of the three dimensional crystalline regions of the Emeraldine-II polymers, in the base EB-II and in the salt ES-II forms, by means of first principle Car-Parrinello molecular dynamics. We have first provided the detailed morphology of each chain in terms of bond lengths, bond angles, torsion angles, and twisting and bending of the chain backbone. We have then addressed the effects on the single chains due to packing of the polymer chains which form the 3D crystal. Starting from the \u201coptimal\u201d configuration obtained for the base form of the polymer, we have studied the process of protonation with HCl of the imminic nitrogens of the chains and the effect of the counterions between neighboring chains, produced by the oxidation of the polymer. In particular we have been able to give a vivid picture of the migration of the Clorine atoms along the polymer backbone and to provide a sound theoretical interpretation of the experimental X-ray diffraction spectra

Structural, electronic, and optical properties of a prototype columnar discotic liquid crystal

Structural, electronic, and optical properties calculated for an isolated infinite column of hexakis-hydroxy-triphenylene HAT0 molecules are presented. This system is intended as a prototype model of columnar discotic liquid crystals since HAT0 is the first member of the discogenic hexakis-n-alkyloxy-triphenylenes HATn series; the single-column approximation can be adopted in view of the peculiar nanoseparation of the columns characteristic of these mesophases. Structural optimization of the system has been performed using Car\u2013Parrinello molecular dynamics techniques. Kohn\u2013Sham orbitals, density of states, and electronic energies have been calculated on the optimized structure of the infinite column and implemented in the quantum expression of the transverse dielectric function. The optical absorption spectrum calculated from the dielectric function has been discussed in comparison to a measured absorption spectrum of HAT5 in columnar discotic liquid crystal phase. Optical absorption spectra of short columns of a few HAT0 molecules arranged as in the infinite column have been calculated using the ZINDO method. These spectra are in good agreement with measured absorption spectra of HAT5 and HAT6 in organic solvents, a fact that supports the proposed columnar arrangement of the discotic molecules in these solvents. We also give an estimate of the electronic transport properties of a small column of HAT0 molecules by calculating with the nonequilibrium Green\u2019s function method, the current/voltage characteristic of a model system in which a few HAT0 molecules have been inserted between two gold electrodes under the bias of an external potential

HCl-doped conducting Emeraldine polymer by ab-initio Car Parrinello Molecular Dynamics

The HCl acidification process of the three dimensional Emeraldine Base II polymer is studied by means of Car-Parrinello Molecular Dynamics. Following X-ray experimental suggestions we have considered different starting simulation cells belonging either to the P21221 or to the Pc2a space groups. We have found that only the Pc2a primitive cell with chlorine counterions inserted in polaronic positions can reproduce the X-ray experimental spectrum; moreover, the backbone nitrogen atoms of each polymer chain are arranged on a plane, all the rings of the protonated chains assume the same character intermediate between benzenoid and quinoid, and all the nitrogen imine groups become nitrogen amine groups. Our study definitely supports the metallic character of the ES-II polymer as due to the Pi-electrons along each chain with chlorine counterions in polaronic arrangement. The presence of Cl- - -H---N hydrogen bonds has also been highlighted

On the B-spline effective completeness

Effective completeness of B-splines, defined as the capability of approaching completeness without compromising the positive definite character of the corresponding superposition matrix, is investigated. A general result on the limit solution of the spectrum of B-splines superposition matrices has been obtained for a large class of knots grids. The result has been tested on finite dimensional cases using both constant and random knots spacings (uniform distribution in [0; 1]). The eigenvalue distribution for random spacings is found not to exhibit any large deviation from that for constant spacings. As an example of system which takes huge advantage of a non uniform grid of knots, we have computed few hundreds of hydrogen Rydberg states obtaining accuracy comparable to the machine accuracy. The obtained results give solid ground to the recognized efficiency and accuracy of the the B-spline sets when used in atomic physics calculations

The photodouble ionization of the ns shell of rare gases

The triple differential cross sections, TDCS, for the photodouble ionization of He, Ne, Ar and Xe leading to the He2+ (1s0 1Se), Ne2+(2s02p6 1Se), Ar2+(3s03p6 1Se)and Xe2+(5s05p6 1Se) states have been measured at about 20 eV above their respective thresholds with the two photoelectrons equally sharing the excess energy. The experimental data are analysed using a novel parametrization recently proposed. A satisfactory representation of the shape of the measured TDCS and their dependence on the increasing correlation with the principal quantum number of the ionised orbital is achieved

Anisotropic molecular packing of soluble C60 fullerenes in hexagonal nanocrystals obtained by solvent vapour annealing

We describe the self-assembly of soluble, chemically modified fullerene [6,6]-phenyl C61 butyric acid methyl ester (PCBM) into a new crystalline phase where the C60 moieties are arranged in parallel layers. Minimum C60 center-to-center distance is 10A\ub0 within the layers, and up to 15A\ub0 perpendicular to the layers. Highly anisotropic, mesoscopic hexagonal crystals of this material, with a lateral size of many microns and a thickness below 1 lm, are obtained from chloroform solution by solvent vapor annealing, and characterized by optical microscopy and X-ray diffraction. The crystalline structure is deduced combining experimental data with molecular modeling and ab initio calculations. The large difference in C60\u2013C60 spacing indicates a high anisotropy in electrical and charge transport properties of this new phase