59 research outputs found

    Approximating electronically excited states with equation-of-motion linear coupled-cluster theory

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    A new perturbative approach to canonical equation-of-motion coupled-cluster theory is presented using coupled-cluster perturbation theory. A second-order M{\o}ller-Plesset partitioning of the Hamiltonian is used to obtain the well known equation-of-motion many-body perturbation theory (EOM-MBPT(2)) equations and two new equation-of-motion methods based on the linear coupled-cluster doubles (EOM-LCCD) and linear coupled-cluster singles and doubles (EOM-LCCSD) wavefunctions. This is achieved by performing a short-circuiting procedure on the MBPT(2) similarity transformed Hamiltonian. These new methods are benchmarked against very accurate theoretical and experimental spectra from 25 small organic molecules. It is found that the proposed methods have excellent agreement with canonical EOM-CCSD state for state orderings and relative excited state energies as well as acceptable quantitative agreement for absolute excitation energies compared with the best estimate theory and experimental spectra.Comment: 9 pages 3 figure

    A route to improving RPA excitation energies through its connection to equation-of-motion coupled cluster theory

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    We revisit the connection between equation-of-motion coupled cluster (EOM-CC) and random phase approximation (RPA) explored recently by Berkelbach [J. Chem. Phys. 149, 041103 (2018)] and unify various methodological aspects of these diverse treatment of ground and excited states. The identity of RPA and EOM-CC based on the ring coupled cluster doubles is established with numerical results which was proved previously on theoretical grounds. We then introduce new approximations in EOM-CC and RPA family of methods, assess their numerical performance and explore a way to reap the benefits of such a connection to improve on excitation energies. Our results suggest that addition of perturbative corrections to account for double excitations and missing exchange effects could result in significantly improved estimates

    A route to improving RPA excitation energies through its connection to equation-of-motion coupled cluster theory

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    We revisit the connection between equation-of-motion coupled cluster (EOM-CC) and random phase approximation (RPA) explored recently by Berkelbach [J. Chem. Phys. 149, 041103 (2018)] and unify various methodological aspects of these diverse treatments of ground and excited states. The identity of RPA and EOM-CC based on the ring coupled cluster doubles is established with numerical results, which was proved previously on theoretical grounds. We then introduce new approximations in EOM-CC and RPA family of methods, assess their numerical performance, and explore a way to reap the benefits of such a connection to improve on excitation energies. Our results suggest that addition of perturbative corrections to account for double excitations and missing exchange effects could result in significantly improved estimates

    Forest landscape ecology and global change: what are the next steps?

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    In this chapter, we summarize current trends and challenges and future research directions in forest landscape ecology and in management related to global change. We discuss the available knowledge in forest landscape ecology and the possibilities of using this knowledge to support management under changing conditions. We also discuss the forest sector’s preparedness to deal with changes in management and how forest landscape ecology can guide this management. Forest landscape ecology has gathered substantial knowledge on patterns, processes, tools, and methods that can support forest and landscape management during changing scenarios. We recognize that existing knowledge is incomplete and that a substantial portion of our knowledge is uncertain, that variability in landscape conditions and various forms of error compound the problem, that we still lack considerable knowledge in some fields, and that there are likely to be knowledge gaps we are not aware of. We nonetheless face the challenge of responding to change based on the available knowledge

    Forest landscapes and global change: Challenges for research and management

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    Climate change, urban sprawl, abandonment of agriculture, intensifi cation of forestry and agriculture, changes in energy generation and use, expansion of infrastructure networks, habitat destruction and degradation, and other drivers and pressures of change are occurring at increasing rates globally. They affect ecological patterns and processes in forest landscapes and modify ecosystem services derived from those ecosystems. Consequently, the landscapes that are rapidly changing in response to these pressures present many new challenges to scientists and managers. Although it is not uncommon to encounter the terms “global change” and “landscape” together in the ecological literature, there has been no adequate global analysis of drivers of change in forest landscapes and their ecological consequences. Providing such an analysis is the goal of this volume: an exploration of the state of knowledge of global changes in forested landscapes, with an emphasis on their causes and effects, and the challenges faced by researchers and land managers who must cope with these changes. This book was based on the IUFRO Landscape Ecology Working Group International Conference that took place in Bragança, Portugal, in September 2010 under the theme “Forest Landscapes and Global Change: New Frontiers in Management, Conservation and Restoration”. The event brought together more than 300 landscape ecologists from almost 50 countries and 5 continents, who came to expand their knowledge and awareness of global changes in forest landscapes. We hope that the syntheses in this book, prepared by a diverse group of scientists who participated in the conference, will enhance the global understanding of a range of topics relevant to change in forest landscapes and stimulate new research to answer the questions raised by these authors. First, we introduce the broad topic of forest landscape ecology and global change. This is followed by chapters that identify and describe major agents of landscape change: climate (Iverson et al.), wildfi re (Rego and Silva), and human activities (Farinaci et al.). The next chapters address implications of change for ecosystem services (Marta-Pedroso et al.), carbon fl uxes (Chen et al.), and biodiversity conservation (Saura et al.). A subsequent chapter describes methodologies for detecting and monitoring landscape changes (Gómez-Sanz et al.) and is followed by a chapter that highlights the many challenges facing forest landscape managers amidst global change (Coulson et al.). Finally, we present a summary and a synthesis of the main points presented in the book (Azevedo et al.). Each chapter was inspired by the research experience of the authors, augmented by a review and synthesis of the global scientifi c literature on relevant topics, as well as critical input from multiple peer reviewers. The intended audience for this book includes graduate students, educators, and researchers in landscape ecology, conservation biology, and forestry, as well as land-use planners and managers. We trust that the wide range of topics, addressed from a global perspective by a geographically diverse group of contributing authors from Europe, North America, and South America, will make this volume attractive to a broad readership.We gratefully acknowledge the following peer reviewers who helped improve the content of this book: Berta Martín, Bill Hargrove, Bob Keane, Colin Beier, Don McKenzie, Eric Gustafson, Franz Gatzweiler, Geoff Henebry, Kurt Riitters, Maria Esther Núñez, Michael Ter-Mikaelian, Tom Nudds, and Yolanda Wiersma. As well, we thank Geoff Hart for assistance with editing and Janet Slobodien and Zachary Romano for assistance with publishing. We also thank FCT (the Foundation for Science and Technology, Portugal), CIMO (the Mountain Research Centre, Portugal), and IPB (the Polytechnic Institute of Bragança, Portugal) for their support during the preparation of this volume.info:eu-repo/semantics/publishedVersio

    Reference Dependence of the Two-determinant Coupled-cluster Method for Triplet and Open-shell Singlet States of Biradical Molecules

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    We study the performance of the two-determinant (TD) coupled-cluster (CC) method which, unlike conventional ground-state single-reference (SR) CC methods, can, in principle, provide a naturally spin-adapted treatment of the lowest-lying open-shell singlet (OSS) and triplet electronic states. Various choices for the TD-CC reference orbitals are considered, including those generated by the multi-configurational self-consistent field method. Comparisons are made with the results of high-level SR-CC, equation-of-motion (EOM) CC, and multi-reference EOM calculations performed on a large test set of over 100 molecules with low-lying OSS states. It is shown that in cases where the EOMCC reference function is poorly described, TD-CC can provide a significantly better quantitative description of OSS total energies and OSS-triplet splittings

    Single-Reference Coupled Cluster Theory for Multi-Reference Problems

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    Coupled cluster (CC) theory is widely accepted as the most accurate and generally applicable approach in quantum chemistry. CC calculations are usually performed with single Slater-determinant references, e.g., canonical Hartree-Fock (HF) wavefunctions, though any single determinant can be used. This is an attractive feature because typical CC calculations are straightforward to apply, as there is no potentially ambiguous user input required. On the other hand, there can be concern that CC approximations give unreliable results when the reference determinant provides a poor description of the system of interest, i.e., when the HF or any other single determinant ground state has a relatively low weight in the full CI expansion. However, in many cases, the reported “failures” of CC can be attributed to an unfortunate choice of reference determinant, rather than intrinsic shortcomings of CC itself. This is connected to well-known effects like spin-contamination, wavefunction instability, and symmetry-breaking. In this contribution, a particularly difficult singlet/triplet splitting problem in two phenyldinitrene molecules is investigated, where CC with singles, doubles and perturbative triples [CCSD(T)] was reported to give poor results. This is analyzed by using different reference determinants for CCSD(T), as well as performing higher level CCSDT-3 and CCSDT calculations. We show that doubly electron attached and doubly ionized equation-of-motion (DEA/DIP-EOM) approaches are powerful alternatives for treating such systems. These are operationally single-determinant methods that adequately take the multi-reference nature of these molecules into account. Our results indicate that CC remains a powerful tool for describing systems with both static correlation and dynamic correlation, when pitfalls associated with the choice of the reference determinant are avoided

    Monte Carlo configuration interaction with perturbation corrections for dissociation energies of first row diatomic molecules: C-2, N-2, O-2, CO, and NO

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    Dissociation energies for the diatomic molecules C2, N2, O2, CO, and NO are estimated using the Monte Carlo configuration interaction (MCCI) and augmented by a second order perturbation theory correction. The calculations are performed using the correlation consistent polarized valence “triple zeta” atomic orbital basis and resulting dissociation energies are compared to coupled cluster calculations including up to triple excitations (CCSDT) and Full Configuration Interaction Quantum Monte Carlo (FCIQMC) estimates. It is found that the MCCI method readily describes the correct behavior for dissociation for the diatomics even when capturing only a relatively small fraction (∼80%) of the correlation energy. At this level only a small number of configurations, typically O(103) from a FCI space of dimension O(1014), are required to describe dissociation. Including the perturbation correction to the MCCI estimates, the difference in dissociation energies with respect to CCSDT ranges between 1.2 and 3.1 kcal/mol, and the difference when comparing to FCIQMC estimates narrows to between 0.5 and 1.9 kcal/mol. Discussions on MCCI's ability to recover static and dynamic correlations and on the form of correlations in the electronic configuration space are presented

    HIGH RATE BIO METHANATION OF MUNICIPAL SOLID WASTE

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    Growing urbanization and industrialization have led to the generation of large quantities of solidwastes, which can be broadly classified as MSW (Municipal Solid Waste) and ISW (Industrial SolidWaste). Among the options available for solid waste management, biomethanation appears to be themost desirable because it is a technology with triple benefits: yields biogas which can replaceconventional fuels, provides digested sludge which can be used as a soil conditioner and manages theenvironment.This work involved the development of a three-stage leachate based bioreactor system for solid wastemanagement. The first stage involves extraction of high organic strength (high in chemical oxygendemand - COD) liquid called leachate from the solids. This stage is known as the hydrolysing stage.Acid formation takes place in the second or the acidification stage in a separate reactor known as theacidification reactor. Biogas is generated by treating the acidified leachate in an upflow anaerobicfilter (AF) reactor in the third (or the methanation) stage.Three stage anaerobic systems arc increasingly finding its place in biogas generation. Two kineticallydissimilar groups of bacteria, acetogenes and methogencs arc physically separated in acetogenicreactor and methanogenic reactor. This separation allows optimization of both acetogenesis andmethanogenesis stages making the process control easier.In this method, the organic solid waste is cut into small pieces, fed into the hydrolysing reactor withwater and circulated at a fixed rate to wash off organic acids formed until high organic strength isobtained. This is then fed in to the acidification reactor. The acidification phase has retention time ofG days. Anaerobic conditions prevail inside the reactor during the whole process. The residue leftinside from the first reactor is subjected to drying for manure preparation.The final concentrated leachate obtained after 6 days is fed in to the AF reactor for methanation. Thisphase has retention time of 5 days. The microbial consortia present in the AF sludge destroys highamount of COD, forming biogas comprising of 65% - 75% methane. This system is economicallyviable, the total space and the time required for the process is low compared to conventional singlephaseprocesses that takes 30-40 days. The system has low water consumption because of the reuse ofthe AF reactor overflow to the acidification reactor.
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