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We discuss a variety of many-body approaches, within effective-mass and $\mathbf{k}\cdot\mathbf{p}$ envelope-function formalisms, for calculating correlated single excitons in semiconductor nanocrystals (NCs) to all orders in the electron-hole Coulomb interaction. These approaches are applied to NCs of the lead-halide perovskite CsPbBr$_{3}$, which typically present excitons in intermediate confinement with physical observables often strongly renormalized by correlation (e.g., radiative decay rate enhanced by a factor of about 7 relative to a mean-field approach, for a NC of edge length 11 nm). The many-body methods considered include the particle-hole Bethe-Salpeter equation, configuration interaction with single excitations, and the random-phase approximation with exchange (RPAE), which are shown to be closely related to each other but to treat $\mathbf{k}\cdot\mathbf{p}$ corrections differently, with RPAE being the most complete method. The methods are applied to calculate the correlation energy, the radiative lifetime, and the long-range Coulomb contribution to the fine structure of the ground-state exciton. In the limit of large NC sizes, the numerical results are shown to agree well with analytical results for this limit, where these are known. Correlated excited states of the single exciton are used to calculate the one-photon absorption cross section; the shape of the resulting cross-section curve (versus laser wavelength) at threshold and up to an excitation energy of about 1 eV is in good agreement with experimental cross sections. The equations for the methods are explicitly adapted to spherical symmetry (involving radial integrals and angular factors) and in this form permit a rapid computation for systems in intermediate confinement., Comment: 15 pages, 8 figures; corrected typos; minor additions to Secs. II, III A, III D, and III E

The one-photon absorption cross section of nanocrystals (NCs) of the inorganic perovskite CsPbBr$_{3}$ is studied theoretically using a multiband $\mathbf{k}\cdot\mathbf{p}$ envelope-function model combined with a treatment of intercarrier correlation by many-body perturbation theory. A confined exciton is described first within the Hartree-Fock (HF) approximation, and correlation between the electron and hole is then included in leading order by computing the first-order vertex correction to the electron-photon interaction. The vertex correction is found to give an enhancement of the near-threshold absorption cross section by a factor of up to 4 relative to the HF (mean-field) value of the cross section, for NCs with an edge length $L=9$-12 nm (regime of intermediate confinement). The vertex-correction enhancement factors are found to decrease with increasing exciton energy; the absorption cross section for photons of energy $\omega=3.1$ eV (about 0.7 eV above threshold) is enhanced by a factor of only 1.4-1.5 relative to the HF value. The $\mathbf{k}\cdot\mathbf{p}$ corrections to the absorption cross section are also significant; they are found to increase the cross section at an energy $\omega=3.1$ eV by about 30% relative to the value found in the effective-mass approximation. The theoretical absorption cross section at $\omega=3.1$ eV, assuming a Kane parameter $E_{P}=20$ eV, is found to be intermediate among the set of measured values (which vary among themselves by nearly an order of magnitude) and to obey a power-law dependence $\sigma^{(1)}(\omega)\propto L^{2.9}$ on the NC edge length $L$, in good agreement with experiment. The dominant contribution to the theoretical exponent 2.9 is shown to be the density of final-state excitons. The main theoretical uncertainty in these calculations is in the value of the Kane parameter $E_{P}$., Comment: 15 pages, 8 figures

We calculate the shift in emission frequency of the trion and biexciton (relative to that of the single exciton) for nanocrystals (NCs) of inorganic perovskites CsPbBr3 and CsPbI3. The calculations use an envelope-function k.p model combined with self-consistent Hartree-Fock and a treatment of the intercarrier correlation energy in the lowest (second) order of many-body perturbation theory. The carriers in the trion and biexciton are assumed to have relaxed nonradiatively to the ground state at the band edge before emission occurs. The theoretical trion shifts for both CsPbBr3 and CsPbI3 are found to be in fair agreement with available experimental data, which include low-temperature single-dot measurements, though are perhaps systematically small by a factor of order 1.5, which can plausibly be explained by a combination of a slightly overestimated dielectric constant and omitted third- and higher-order terms in the correlation energy. Taking this level of agreement into account, we estimate that the ground-state biexciton shift for CsPbBr3 is a redshift of order 10-20 meV for NCs with an edge-length of 12 nm. This value is intermediate among the numerous high-temperature measurements on NCs of CsPbBr3, which vary from large redshifts of order 100 meV to blueshifts of several meV., Comment: 12 pages, 4 figures

We use the lac operon in Escherichia coli as a prototype system to illustrate the current state, applicability, and limitations of modeling the dynamics of cellular networks. We integrate three different levels of description -molecular, cellular, and that of cell population- into a single model, which seems to capture many experimental aspects of the system.

The surface plasmon in simple metal clusters is red-shifted from the Mie frequency, the energy shift being significantly larger than the usual spill-out correction. Here we develop a variational approach to the RPA collective excitations. Using a simple trial form, we obtain analytic expressions for the energy shift beyond the spill-out contribution. We find that the additional red shift is proportional to the spill-out correction and can have the same order of magnitude., Comment: 17 pages and 2 figures

The fission of highly charged sodium clusters with fissilities X>1 is studied by {\em ab initio} molecular dynamics. Na_{24}^{4+} is found to undergo predominantly sequential Na_{3}^{+} emission on a time scale of 1 ps, while Na_{24}^{Q+} (5 \leq Q \leq 8) undergoes multifragmentation on a time scale \geq 0.1 ps, with Na^{+} increasingly the dominant fragment as Q increases. All singly-charged fragments Na_{n}^{+} up to size n=6 are observed. The observed fragment spectrum is, within statistical error, independent of the temperature T of the parent cluster for T \leq 1500 K. These findings are consistent with and explain recent trends observed experimentally., Comment: To appear in Physical Review Letters