We present an approach for calculating coarse-grained angle-resolved effective pair potentials for uniaxial molecules. For integrating out the intramolecular degrees of freedom we apply umbrella sampling and steered dynamics techniques in atomistically-resolved molecular dynamics (MD) computer simulations. Throughout this study we focus on disk-like molecules such as coronene. To develop the methods we focus on integrating out the van-der-Waals and intramolecular interactions, while electrostatic charge contributions are neglected. The resulting coarse-grained pair potential reveals a strong temperature and angle dependence. In the next step we fit the numerical data with various Gay-Berne-like potentials to be used in more efficient simulations on larger scales. The quality of the resulting coarse-grained results is evaluated by comparing their pair and many-body structure as well as some thermodynamic quantities self-consistently to the outcome of atomistic MD simulations of many-particle systems. We find that angle-resolved potentials are essential not only to accurately describe crystal structures but also for fluid systems where simple isotropic potentials start to fail already for low to moderate packing fractions. Further, in describing these states it is crucial to take into account the pronounced temperature dependence arising in selected pair configurations due to bending fluctuations.

In this article, we present and compare two different, coarse-grained approaches to model electrostatic interactions of disc-shaped aromatic molecules, specifically coronene. Our study builds on our previous work [T. Heinemann et al., J. Chem. Phys. 141, 214110 (2014)], where we proposed, based on a systematic coarse-graining procedure starting from the atomistic level, an anisotropic effective (Gay-Berne-like) potential capable of describing van der Waals contributions to the interaction energy. To take into account electrostatics, we introduce, first, a linear quadrupole moment along the symmetry axis of the coronene disc. The second approach takes into account the fact that the partial charges within the molecules are distributed in a ring-like fashion. We then reparametrize the effective Gay-Berne-like potential such that it matches, at short distances, the ring-ring potential. To investigate the validity of these two approaches, we perform many-particle molecular dynamics simulations, focusing on the crystalline phase (karpatite) where electrostatic interaction effects are expected to be particularly relevant for the formation of tilted stacked columns. Specifically, we investigate various structural parameters as well as the melting transition. We find that the second approach yields consistent results with those from experiments despite the fact that the underlying potential decays with the wrong distance dependence at large molecule separations. Our strategy can be transferred to a broader class of molecules, such as benzene or hexabenzocoronene.

We investigate the structural properties of a two-dimensional system of ellipsoidal particles carrying a linear quadrupole moment in their center. These particles represent a simple model for a variety of uncharged, non-polar conjugated organic molecules. Using optimization tools based on ideas of evolutionary algorithms, we first examine the ground state structures as we vary the aspect ratio of the particles and the pressure. Interestingly, we find, besides the intuitively expected T-like configurations, a variety of complex structures, characterized with up to three different particle orientations. In an effort to explore the impact of thermal fluctuations, we perform constant-pressure molecular dynamics simulations within a range of rather low temperatures. We observe that ground state structures formed by particles with a large aspect ratio are in particular suited to withstand fluctuations up to rather high temperatures. Our comprehensive investigations allow for a deeper understanding of molecular or colloidal monolayer arrangements under the influence of a typical electrostatic interaction on a coarse-grained level.

The Metro Wastewater Reclamation District (Metro District) provides wholesale wastewater transmission and treatment service to 55 local governments in the Denver metropolitan area. Solids processing at the Central Treatment Plant (CTP) consists of dissolved air flotation (DAF) thickening of waste activated sludge (WAS), anaerobic digestion of blended primary sludge and thickened WAS, dewatering with high-solids centrifuges, land application at the Metro District's METROGRO Farm located approximately 100 miles east of Denver, and composting of a portion of the digested biosolids. Prior to this project, the Metro District had 10 anaerobic digesters operated in parallel in a high-rate mesophilic digestion process. Based on flow and load projections, a capacity expansion of the process was necessary to meet the needs of the Metro District.CH2M HILL was retained by the Metro District to evaluate ways to meet its future sludge digestion needs and provide additional digestion capacity through the design year 2020. Following evaluation of numerous alternatives, the decision was made to construct two additional digesters (Digesters 11 and 12) to provide the additional needed capacity. In addition, the decision was made to provide the flexibility to operate the digestion process in either a single-stage mode or a two-phase (acid/gas) mode. Key components of the construction project included:• Two new anaerobic digesters, designed to operate as either acid, methane, or combined modes as part of either a single-stage or two-phase process. • A new Digester Control Building, improvements to the existing Digester Mall, and improvements to the existing anaerobic digesters. • Improvements to the hot water system. Hot water is created by waste heat from the CTP's Cogeneration system, with three natural gas fired boilers serving as back up. • Improvements to the digester gas handling system, including four new waste gas flares.• Acid sludge pumps and digested sludge pumps.• Process Control and instrumentation upgrades.The innovative two-phase process separates the two biological steps of anaerobic digestion into separate vessels for maximum efficiency. The two-phase system consists of highly loaded "acid phase" digesters followed by lightly loaded "gas phase" or "methane" digesters. The first stage provides hydrolysis and acidification pretreatment, and the second stage maximizes gas production. The two-phase process is anticipated to result in improved volatile solids reduction, higher gas production, and improved process stability. The new digesters were placed into operation in late 2005 and the digestion process was converted to two-phase in February 2006. 6752 WEFTEC®.07This paper will briefly summarize the main components of the project and explain the two-phase digestion process and how it is operated. It will focus primarily on describing the lessons learned in starting up the new facilities and the process optimization efforts completed after startup. It will relate the impacts of a separate DAF thickening and primary slud...

We present a coarse-graining strategy for reducing the number of particle species in mixtures to achieve a simpler system with higher diffusion while preserving the total particle number and characteristic dynamic features. As a system of application, we chose the bidisperse Lennard-Jones-like mixture, discovered by Kob and Andersen [Phys. Rev. Lett. 73, 1376 (1994)], possessing a slow dynamics due to the fluid’s multi-component character with its apparently unconventional choice for the pair potential of the type-A–type-B arrangement. We further established in a so-formed coarse-grained and temperature-independent monodisperse system an equilibrium structure with a radial distribution function resembling its mixture counterpart. This one-component system further possesses similar dynamic features such as glass transition temperature and critical exponents while subjected to Newtonian mechanics. This strategy may finally lead to the manufacturing of new nanoparticle/colloidal fluids by experimentally modeling only the outcoming effective pair potential(s) and no other macroscopic quantity.