Starting from our earlier model [J. Chern. Phys. 66, 1496] a simple expression is derived for the radial dependent damping functions for the individual dispersion coefficients C Zn for arbitrary even orders 2n. The damping functions are only a function of the Born-Mayer range parameter b and thus can be applied to all systems for which this is known or can be estimated. For H( IS )-H( IS ) the results are in almost perfect agreement with the very accurate recent ab initio damping functions of Koide, Meath, and Allnatt. Comparisons with less accurate previous calculations for other systems also show a satisfactory agreement. By adding a Born-Mayer repulsive term [A exp( -bR )] to the damped dispersion potential, a simple universal expression is obtained for the well region of the atom-atom van der Waals potential with only five essential parameters A, b, C 6 , Cg, and C IO ' The model has been tested for the following representative systems: Hz 3 I, He z , and Ar z as well as NaK 3 I and LiHg, which include four chemically different types of van der Waals interactions for which either very precise theoretical or experimental data is available. For each system the ab initio dispersion coefficients together with the well-known parameters E and R m were used to determine A and b from the model potential.With these values the reduced potentials were calculated and found to agree with the experimental potentials to better than 1 % and always less than the experimental uncertainties. Some of the implications of the new model are discussed.

The interatomic van der Waals potentials for all the possible 21 homogeneous and heterogeneous pairs of rare gas atoms including radon are determined using the Tang–Toennies potential model and a set of previously derived combining rules. The three dispersion coefficients and the two Born–Mayer parameters needed for calculating the potential curves are listed.

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BackgroundChloroflexus aurantiacus is a thermophilic filamentous anoxygenic phototrophic (FAP) bacterium, and can grow phototrophically under anaerobic conditions or chemotrophically under aerobic and dark conditions. According to 16S rRNA analysis, Chloroflexi species are the earliest branching bacteria capable of photosynthesis, and Cfl. aurantiacus has been long regarded as a key organism to resolve the obscurity of the origin and early evolution of photosynthesis. Cfl. aurantiacus contains a chimeric photosystem that comprises some characters of green sulfur bacteria and purple photosynthetic bacteria, and also has some unique electron transport proteins compared to other photosynthetic bacteria.MethodsThe complete genomic sequence of Cfl. aurantiacus has been determined, analyzed and compared to the genomes of other photosynthetic bacteria.ResultsAbundant genomic evidence suggests that there have been numerous gene adaptations/replacements in Cfl. aurantiacus to facilitate life under both anaerobic and aerobic conditions, including duplicate genes and gene clusters for the alternative complex III (ACIII), auracyanin and NADH:quinone oxidoreductase; and several aerobic/anaerobic enzyme pairs in central carbon metabolism and tetrapyrroles and nucleic acids biosynthesis. Overall, genomic information is consistent with a high tolerance for oxygen that has been reported in the growth of Cfl. aurantiacus. Genes for the chimeric photosystem, photosynthetic electron transport chain, the 3-hydroxypropionate autotrophic carbon fixation cycle, CO2-anaplerotic pathways, glyoxylate cycle, and sulfur reduction pathway are present. The central carbon metabolism and sulfur assimilation pathways in Cfl. aurantiacus are discussed. Some features of the Cfl. aurantiacus genome are compared with those of the Roseiflexus castenholzii genome. Roseiflexus castenholzii is a recently characterized FAP bacterium and phylogenetically closely related to Cfl. aurantiacus. According to previous reports and the genomic information, perspectives of Cfl. aurantiacus in the evolution of photosynthesis are also discussed.ConclusionsThe genomic analyses presented in this report, along with previous physiological, ecological and biochemical studies, indicate that the anoxygenic phototroph Cfl. aurantiacus has many interesting and certain unique features in its metabolic pathways. The complete genome may also shed light on possible evolutionary connections of photosynthesis.