Phase diagram of solution of oppositely charged polyelectrolytes

We study a solution of long polyanions (PA) with shorter polycations (PC) and focus on the role of Coulomb interaction. A good example is solutions of DNA and PC which are widely studied for gene therapy. In the solution, each PA attracts many PCs to form a complex. When the ratio of total charges of PA and PC in the solution, $x$, equals to 1, complexes are neutral and they condense in a macroscopic drop. When $x$ is far away from 1, complexes are strongly charged. The Coulomb repulsion is large and free complexes are stable. As $x$ approaches to 1, PCs attached to PA disproportionate themselves in two competing ways. One way is inter-complex disproportionation, in which PCs make some complexes neutral and therefore condensed in a macroscopic drop while other complexes become even stronger charged and stay free. The other way is intra-complex disproportionation, in which PCs make one end of a complex neutral and condensed in a small droplet while the rest of the complex forms a strongly charged tail. Thus each complex becomes a "tadpole". These two ways can also combine together to give even lower free energy. We get a phase diagram of PA-PC solution in a plane of $x$ and inverse screening radius of the monovalent salt, which includes phases or phase coexistence with both kinds of disproportionation.Comment: 29 pages, 10 figures. Major change in results and tex

Backward diode composed of a metallic and semiconducting nanotube

The conditions necessary for a nanotube junction connecting a metallic and semiconducting nanotube to rectify the current are theoretically investigated. A tight binding model is used for the analysis, which includes the Hartree-Fock approximation and the Green's function method. It is found that the junction has a behavior similar to the backward diode if the gate electrode is located nearby and the Fermi level of the semiconducting tube is near the gap. Such a junction would be advantageous since the required length for the rectification could be reduced.Comment: 4 pages, RevTeX, uses epsf.st

Vibrational signatures for low-energy intermediate-sized Si clusters

We report low-energy locally stable structures for the clusters Si20 and Si21. The structures were obtained by performing geometry optimizations within the local density approximation. Our calculated binding energies for these clusters are larger than any previously reported for this size regime. To aid in the experimental identification of the structures, we have computed the full vibrational spectra of the clusters, along with the Raman and IR activities of the various modes using a recently developed first-principles technique. These represent, to our knowledge, the first calculations of Raman and IR spectra for Si clusters of this size

Rearrangements of Ammonium Nitrate Cluster Ions with High Internal Energy

Sputtering of condensed-phase ammonium nitrate yields an extensive distribution of negative cluster ions of the form [(NH4NO3)nNO3]-, n≥3. Collision-induced dissociation of mass-selected cluster ions suggests that the first two members of the series, n = 1 and n = 2, are missing because the ions rearrange to lose one or more ammonia molecules. Gradient-corrected density-functional calculations show that NH4NO3 is strongly hydrogen bonded and that [(NH4NO3)NO3]- has no hydrogen bonds, consistent with this ion rearranging to lose NH3 to form the strongly hydrogen-bonded ion [H(NO3)2]-

On the gaussian-type orbitals approach to local density functional theory

Two most pressing practical problems with the linear combination of Gaussian-type orbitals (LCGTO) approach to local density functional (LDF) theory both relate to the treatment of the exchange and correlation potential. They are the problem of (effective or direct) numerical integration on a grid of points and the general LDF problem of the spin dependence of the exchange and correlation functional. We discuss our recent progress in solving the SCF problems associated with a treatment of the la (only) exchange and correlation functional that uses only analytic integrals, thereby avoiding any grid of points. This advance will allow exact analytic gradients; the first derivative expressions are presented. The complete LDF treatment of elemental clusters is an area in which gradients are indispensable. Without such means to optimize the large number of nuclear degrees of freedom in a thirteen-atom cluster, we have studied three highly symmetric thirteen—atom nickel clusters using the la and the Perdew—Zunger approximations to exchange and correlation in order to see if they differ in their predictions of the magnetism of different isomers of clusters, experimental quantities that will likely soon be measured