Representing potential energy surfaces by high-dimensional neural network potentials.

The development of interatomic potentials employing artificial neural networks has seen tremendous progress in recent years. While until recently the applicability of neural network potentials (NNPs) has been restricted to low-dimensional systems, this limitation has now been overcome and high-dimensional NNPs can be used in large-scale molecular dynamics simulations of thousands of atoms. NNPs are constructed by adjusting a set of parameters using data from electronic structure calculations, and in many cases energies and forces can be obtained with very high accuracy. Therefore, NNP-based simulation results are often very close to those gained by a direct application of first-principles methods. In this review, the basic methodology of high-dimensional NNPs will be presented with a special focus on the scope and the remaining limitations of this approach. The development of NNPs requires substantial computational effort as typically thousands of reference calculations are required. Still, if the problem to be studied involves very large systems or long simulation times this overhead is regained quickly. Further, the method is still limited to systems containing about three or four chemical elements due to the rapidly increasing complexity of the configuration space, although many atoms of each species can be present. Due to the ability of NNPs to describe even extremely complex atomic configurations with excellent accuracy irrespective of the nature of the atomic interactions, they represent a general and therefore widely applicable technique, e.g. for addressing problems in materials science, for investigating properties of interfaces, and for studying solvation processes.

Structure determination of small vanadium clusters by density-functional theory in comparison with experimental far-infrared spectra.

The far-infrared vibrational spectra for charged vanadium clusters with sizes of 3-15 atoms have been measured using infrared multiple photon dissociation of Vn+Ar-->Vn(+)+Ar. Using density-functional theory calculations, we calculated the ground state energy and vibrational spectra for a large number of stable and metastable geometries of such clusters. Comparison of the calculated vibrational spectra with those obtained in the experiment allows us to deduce the cluster size specific atomic structures. In several cases, a unique atomic structure can be identified, while in other cases our calculations suggest the presence of multiple isomers.

A molecular phylogeny of four endangered Madagascar tortoises based on MtDNA sequences.

Four of the five tortoise species in Madagascar, Pyxis arachnoides, P. planicauda, Geochelone radiata, and G. yniphora, are endemic and on the verge of extinction. Their phylogenetic relationships remain controversial and unresolved. Here we address the phylogeny of this group using DNA sequences for the 12S and 16S rDNA and cyt b genes in mitochondrial DNA. As outgroups we used two species of Geochelone, pardalis (mainland Africa) and nigra (Galápagos), as well as a more distant North American tortoise, Gopherus polyphemus. We conclude that the two Pyxis species are sister taxa and are imbedded in the genus Geochelone, rendering this latter genus paraphyletic. There is moderate support for the sister status of the two Madagascar Geochelone and for the monophyletic origin of all four endemics, suggesting a single colonization of the island. The separation of Madagascar from other land masses (90-165 mya) predates the origin of the endemic tortoises (estimated to be 14-22 mya). This suggests founding by rafting, a process known to have occurred with other tortoises. The derived morphological divergence of the Pyxis species in a relatively short period of time (13-20 my) stands in contrast to the notoriously slow rate of morphological evolution in most lineages of Chelonia.

Ozone-monitoring in Mendoza, Argentina: initial results.

A series of continuous ambient tropospheric ozone measurements were taken in Mendoza, Argentina, for a period of one year starting in November 1995. The data obtained were analyzed in terms of diurnal and annual variation. Indications were found of the strong impact of the mountain-valley circulation system, which ventilates and considerably cleans the air in Mendoza. The data are discussed in comparison with air pollution in the German city of Leipzig. In Mendoza, the high concentration of precursors and the strong solar radiation contribute to high levels of ozone. In fact, monitoring reveals considerably lower concentrations than in Leipzig, owing to the diluting effect of local meteorology. The low-level jet is mainly active during the summer. It lowers the peak mid-day ozone concentration and produces a temporary concentration increase at night. The Environmental Protection Agency standard of a maximum one-hour mean concentration of 0.250 mg ozone/m3 (125 ppb) is never reached, and the World Health Organization standard of 0.113 mg ozone/m3 (56.5 ppb) is only rarely exceeded during winter.

Cognitive impulsivity training: the effects of peer teaching.

An investigation was conducted into the effects of peer teaching on two 10 1/2-year-old hospitalized impulsive children. One of these, the subject-teacher, was given cognitive behavioral self-instructional training for four weeks and subsequently prompted to train a peer in the problem-solving skills that he had just learned. Initial treatment effects on the subject-teacher, additive benefits obtained by his teaching a peer, and the treatment effects on the subject-pupil were examined using multiple dependent measures. The procedure was found advantageous to both subject-teacher and subject-pupil. Limitations of single-subject experiments and the need for using multiple measures in assessing treatment gains from self-instructional training to reduce impulsivity are discussed.