A simple parameter-free one-center model potential for an effective one-electron description of molecular hydrogen

For the description of an H2 molecule an effective one-electron model potential is proposed which is fully determined by the exact ionization potential of the H2 molecule. In order to test the model potential and examine its properties it is employed to determine excitation energies, transition moments, and oscillator strengths in a range of the internuclear distances, 0.8 < R < 2.5 a.u. In addition, it is used as a description of an H2 target in calculations of the cross sections for photoionization and for partial excitation in collisions with singly-charged ions. The comparison of the results obtained with the model potential with literature data for H2 molecules yields a good agreement and encourages therefore an extended usage of the potential in various other applications or in order to consider the importance of two-electron and anisotropy effects.Comment: 8 pages, 6 figure

Equations of state of elements based on the generalized Fermi-Thomas theory

The Fermi-Thomas model has been used to derive the equation of state of matter at high pressures and at various temperatures. Calculations have been carried out both without and with the exchange terms. Discussion of similarity transformations lead to the virial theorem and to correlation of solutions for different Z values

Space-based geoengineering: challenges and requirements

The prospect of engineering the Earth's climate (geoengineering) raises a multitude of issues associated with climatology, engineering on macroscopic scales, and indeed the ethics of such ventures. Depending on personal views, such large-scale engineering is either an obvious necessity for the deep future, or yet another example of human conceit. In this article a simple climate model will be used to estimate requirements for engineering the Earth's climate, principally using space-based geoengineering. Active cooling of the climate to mitigate anthropogenic climate change due to a doubling of the carbon dioxide concentration in the Earth's atmosphere is considered. This representative scenario will allow the scale of the engineering challenge to be determined. It will be argued that simple occulting discs at the interior Lagrange point may represent a less complex solution than concepts for highly engineered refracting discs proposed recently. While engineering on macroscopic scales can appear formidable, emerging capabilities may allow such ventures to be seriously considered in the long term. This article is not an exhaustive review of geoengineering, but aims to provide a foretaste of the future opportunities, challenges, and requirements for space-based geoengineering ventures

Nuclear energy for the third millennium

The major energy sources of today are expected to last for only a small fraction of the millennium starting three years hence. In the plans of most people, nuclear energy has been ruled out for four separate reasons: 1. The danger of radioactivity from a reactor accident or from reactor products during a long period after reactor shutdown; 2. The proposed fuels, U-235 and also Pu-239, as obtained by presently available procedures will serve only for a limited duration; 3. Energy from nuclear reactors will be more expensive than costs of present alternatives; 4. The possibility of misusing the products for military purposes is an unacceptable danger. The development described below 1 attempts to meet all four objections. Specifically, we propose a structure as an example of future reactors that is deployed two hundred meters underground in loose and dry earth. The reactor is designed to function for thirty years, delivering electrical power on demand up to a level of thousand electrical megawatts. From the time that the reactor is started to the time of its shutdown thirty years later, the functioning is to be completely automatic. This is an obviously difficult condition to fulfill. The most important factor in making it possible is to design and operate the reactor without moving mechanical parts. At the start, the reactor functions on thermal neutrons within a structure containing uranium enriched in U-235 or having an addition of plutonium. That part of the reactor is to deliver energy for approximately one year after which a neighboring portion of the reactor containing thorium has been converted into Th-233 which rather rapidly decays into fissile U-233. This part of the assembly works on fission by fast neutrons. It will heat-up if insufficient thermal energy is withdrawn from the reactor`s core, under the negative feedback action of engineered-in thermostats. Indeed, these specifically designed thermostatic units absorb neutrons if excessive reactor core heating occurs in order to decrease heat generation and to act like automatic control rods. These units will be described below. After the thorium in a given volume of the reactor`s fuel charge is depleted, an adjacent thorium-containing portion of the fuel charge will have been converted bred into fissile material and is ready to continue the reaction. A schematic representation of this concept is shown in Figure 1. Actually, the thorium `reactors` in this Figure will be merged together into a single reactor system with the nuclear fuel-burning reactions propagating down to the ultimate `reactor` U. (In practice, we consider placing the fuel-igniting charge in the middle of the reactor system`s `fuel stick` and arrange breeding regions on both sides, shown in Figure 3.) After all the thorium in the reactor`s fuel charge has been used up, the reactor is shut down by the first positive action of the operators in thirty years. The residual radioactivity will be sealed within the reactor`s core and thereafter allowed to decay in place. The initially intense radioactivity will leave the reactor products inaccessible and unusable for military purposes except if complicated, expensive and easily observed large-scale operations are performed. Having thereby avoided transportation of fission products and reprocessing significantly reduces cost and hazards

Functional strengthening through synaptic scaling upon connectivity disruption in neuronal cultures

An elusive phenomenon in network neuroscience is the extent of neuronal activity remodeling upon damage. Here, we investigate the action of gradual synaptic blockade on the effective connectivity in cortical networks in vitro. We use two neuronal cultures configurations—one formed by about 130 neuronal aggregates and another one formed by about 600 individual neurons—and monitor their spontaneous activity upon progressive weakening of excitatory connectivity. We report that the effective connectivity in all cultures exhibits a first phase of transient strengthening followed by a second phase of steady deterioration. We quantify these phases by measuring GEFF, the global efficiency in processing network information. We term hyperefficiency the sudden strengthening of GEFF upon network deterioration, which increases by 20–50% depending on culture type. Relying on numerical simulations we reveal the role of synaptic scaling, an activity–dependent mechanism for synaptic plasticity, in counteracting the perturbative action, neatly reproducing the observed hyperefficiency. Our results demonstrate the importance of synaptic scaling as resilience mechanism. Author Summary Neuronal circuits exhibit homeostatic plasticity mechanisms to cope with perturbations or damage. A central mechanism is ‘synaptic scaling,’ a self-organized response in which the strength of neurons’ excitatory synapses is adjusted to compensate for activity variations. Here we present experiments in which the excitatory connectivity of in vitro cortical networks is progressively weakened through chemical action. The spontaneous activity and effective connectivity of the whole network is monitored as degradation progresses, and the capacity of the network for broad information communication is quantified through the global efficiency. We observed that the network responded to the perturbation by strengthening the effective connectivity, reaching a hyperefficient state for moderate perturbations. The study proves the importance of ‘synaptic scaling’ as a driver for functional reorganization and network-wide resilience