Monte-Carlo studies of bosonic van der Waals clusters

In a previous paper (http://www.phys.uri.edu/people/nightingale/publications.html, chem-ph/9406003) we developed a form of variational trial wave function and applied it to van der Waals clusters: five or less atoms of Ar and Ne modeled by the Lennard-Jones potential. In addition, we tested the trial functions for a hypothetical, light atom resembling Ne but with only half its mass. We did not study atoms such as He with larger de Boer parameters, i.e., systems in which the zero point energy plays a more important role relative to the potential energy. This is the main purpose of the present paper. In fact, we study clusters to the very limit where the zero-point energy destroys the ground state as a bound state. A simple picture of this un-binding transition predicts the power law with which the energy vanishes as the de Boer parameter approaches its critical value and the power of the divergence of the the size of the clusters in this limit. Our numerical results are in agreement with these predictions.Comment: text and figures TeX form, 4 page

Van der Waals clusters in the ultra-quantum limit: a Monte Carlo study

Bosonic van der Waals clusters of sizes three, four and five are studied by diffusion quantum Monte-Carlo techniques. In particular we study the unbinding transition, the ultra-quantum limit where the ground state ceases to exist as a bound state. We discuss the quality of trial wave functions used in the calculations, the critical behavior in the vicinity of the unbinding transition, and simple improvements of the diffusion Monte Carlo algorithm.Comment: World Wide Web URL http://www.phys.uri.edu/people/mark_meierovich/visual/Main.html contains an informal presentation with color graphic

Optimization of ground and excited state wavefunctions and van der Waals clusters

A quantum Monte Carlo method is introduced to optimize excited state trial wavefunctions. The method is applied in a correlation function Monte Carlo calculation to compute ground and excited state energies of bosonic van der Waals clusters of upto seven particles. The calculations are performed using trial wavefunctions with general three-body correlations

Localized helium excitations in 4He_N-benzene clusters

We compute ground and excited state properties of small helium clusters 4He_N containing a single benzene impurity molecule. Ground-state structures and energies are obtained for N=1,2,3,14 from importance-sampled, rigid-body diffusion Monte Carlo (DMC). Excited state energies due to helium vibrational motion near the molecule surface are evaluated using the projection operator, imaginary time spectral evolution (POITSE) method. We find excitation energies of up to ~23 K above the ground state. These states all possess vibrational character of helium atoms in a highly anisotropic potential due to the aromatic molecule, and can be categorized in terms of localized and collective vibrational modes. These results appear to provide precursors for a transition from localized to collective helium excitations at molecular nanosubstrates of increasing size. We discuss the implications of these results for analysis of anomalous spectral features in recent spectroscopic studies of large aromatic molecules in helium clusters.Comment: 15 pages, 5 figures, submitted to Phys. Rev.

Spatiotemporal variations of soil reactive nitrogen oxide fluxes across the anthropogenic landscape

Volatile reactive nitrogen oxides (NO y ) are significant atmospheric pollutants, including NO x (nitric oxide [NO] + nitrogen dioxide [NO2]) and NO z (nitrous acid [HONO] + nitric acid [HNO3] + nitrogen trioxide [NO3] + ...). NO y species are products of nitrogen (N) cycle processes, particularly nitrification and denitrification. Biogenic sources, including soil, account for over 50% of natural NO y emissions to the atmosphere, yet emissions from soils are generally not included in atmospheric models as a result of a lack of mechanistic data. This work is a unique investigation of NO y fluxes on a landscape scale, taking a comprehensive set of land-use types, human influence, and seasonality into account to determine large-scale heterogeneity to provide a basis for future modeling and hypothesis generation. By coupling 16S rRNA amplicon sequencing and quantitative polymerase chain reaction, we have linked significant differences in functional potential and activity of nitrifying and denitrifying soil microbes to NO y emissions from soils. Further, we have identified soils subject to increased N deposition that are less microbially active despite increased available N, potentially as a result of poor soil health from anthropogenic pollution. Structural equation modeling suggests human influence on soils to be a more significant effector of soil NO y emissions than land-use type

Interactions of long-term grazing and woody encroachment can shift soil biogeochemistry and microbiomes in savanna ecosystems

Semi-arid grasslands and savannas in the southern Great Plains USA are extensively used for livestock grazing. Over the past century, Juniperus (juniper) and Quercus (oak) species abundance have increased due to intensive grazing and reduced fire frequency. We investigated the interactions between livestock grazing history (none, moderate, heavy) and vegetation cover (grassland, juniper, oak) using a ∼ 70-year grazing experiment in west-central Texas. We explored effects on soil organic carbon (SOC), total nitrogen (TN), total phosphorus (TP), microbial community composition, and function. SOC and TN were 50–150 % higher under juniper and oak compared to grasslands, and 10–30 % lower in grazed vs. ungrazed areas. Vegetation × grazing interaction showed greater SOC and TN loss under oak than juniper or grasslands. Ungrazed controls had higher soil TP than grazed areas, with oak and juniper soils having more TP than grasslands. Bacterial and fungal communities differed between grassland and woody vegetation. Grazing affected only bacterial communities. SOC, TN, TP accounted for differences in community structure. Abundances of genes related to methane, nitrogen, sulfur metabolisms, and dominant fungal trophic modes were linked to soil C, N, P ratios. These findings highlight how long-term livestock grazing and woody plant encroachment influence soil C, N, P cycles, altering soil microbial community structure and function. This study provides insights for savanna ecosystem management and integrating land cover effects into biogeochemical models for global change scenarios