Contribution Of Atomic Energy Research Articles

Accurate and Transferable Machine Learning Potential for Molecular Dynamics Simulation of Sodium Silicate Glasses.

An accurate and transferable machine learning (ML) potential for the simulation of binary sodium silicate glasses over a wide range of compositions (from 0 to 50% Na2O) was developed. The potential energy surface is approximated by the sum of atomic energy contributions mapped by a neural network algorithm from the local geometry comprising information on atomic distances and angles with neighboring atoms using the DeePMD code [Wang, H. Comput. Phys. Commun. 2018, 228, 178-184]. Our model was trained on a large data set of total energies and atomic forces computed at the density functional theory level on structures extracted from classical molecular dynamics (MD) simulations performed at several temperatures from 300 to 3000 K. This allows for the generation of a robust and transferable ML potential applicable over the full compositional range of glass formability at different temperatures that outperforms the empirical potentials available in the literature in reproducing structures and properties such as bond angle distribution, total distribution functions, and vibrational density of state. The generality of the approach enables the future training of a potential with other or more elements allowing for simulations of structures, properties, and behavior of ternary and multicomponent oxide glasses with nearly ab initio accuracy at a fraction of the computational cost.

The role of the IAEA in establishing international standards for the use of atomic energy (retrospective analysis)

Paradoxically, the peaceful use of nuclear energy and all the prospects it opens up for humanity often coexist with the prospect of nuclear proliferation and nuclear war. This difficult perception is understandable: the materials, knowledge, and expertise needed to produce nuclear weapons are generally no different from those needed to produce nuclear energy and conduct nuclear research.Therefore, ensuring the peaceful and safe use of nuclear energy has always been in the center of attention of the international community. The approach is determined by a complex of national and international measures. It is generally accepted that the primary responsibility for regulating the use of nuclear energy rests with national authorities, but it is also clear that other countries may also be affected. Thus, like many other human activities with potentially transboundary consequences, nuclear energy regulation requires a residual, and in some cases joint, responsibility on the part of the international community to ensure the harmonization of standards, coordination and pooling of resources and services, and regulatory cooperation this activity.In this regard, the IAEA and other international and regional organizations play the role of coordination centers. Article II of the IAEA Statute states that the organization shall seek to accelerate and increase the contribution of atomic energy to world peace, health and prosperity and shall provide, as far as possible, assistance upon request. IAEA supervision or control shall not be used to achieve any military objective.International peration in the field of nuclear energy over the past six decades has resulted in legally binding norms and advisory standards and regulations. This article outlines this global legal framework for the safe and peaceful development of nuclear energy. In particular, it concerns nuclear safety, radiation protection, radioactive waste management, transportation of radioactive materials, emergency assistance and planning, civil liability for nuclear damage, physical protection of nuclear material, armed attack on nuclear facilities, safeguards and controls carried out and provided by the IAEA.

An Interacting Quantum Atoms (IQA) and Relative Energy Gradient (REG) Analysis of the Anomeric Effect.

The explanation of the anomeric effect in terms of underlying quantum properties is still controversial almost 70 years after its introduction. Here, we use a method called Relative Energy Gradient (REG), which is able to compute chemical insight with a view to explaining the anomeric effect. REG operates on atomic energy contributions generated by the quantum topological energy decomposition Interacting Quantum Atoms (IQA). Based on the case studies of dimethoxymethane and 2-fluorotetrahydropyran, we show that the anomeric effect is electrostatic in nature rather than governed by hyperconjugation.

A Hessian-based assessment of atomic forces for training machine learning interatomic potentials.

In recent years, many types of machine learning potentials (MLPs) have been introduced, which are able to represent high-dimensional potential-energy surfaces (PESs) with close to first-principles accuracy. Most current MLPs rely on atomic energy contributions given as a function of the local chemical environments. Frequently, in addition to total energies, atomic forces are also used to construct the potentials, as they provide detailed local information about the PES. Since many systems are too large for electronic structure calculations, obtaining reliable reference forces from smaller subsystems, such as molecular fragments or clusters, can substantially simplify the construction of the training sets. Here, we propose a method to determine structurally converged molecular fragments, providing reliable atomic forces based on an analysis of the Hessian. The method, which serves as a locality test and allows us to estimate the importance of long-range interactions, is illustrated for a series of molecular model systems and the metal-organic framework MOF-5 as an example for a complex organic-inorganic hybrid material.

Four Generations of High-Dimensional Neural Network Potentials.

Since their introduction about 25 years ago, machine learning (ML) potentials have become an important tool in the field of atomistic simulations. After the initial decade, in which neural networks were successfully used to construct potentials for rather small molecular systems, the development of high-dimensional neural network potentials (HDNNPs) in 2007 opened the way for the application of ML potentials in simulations of large systems containing thousands of atoms. To date, many other types of ML potentials have been proposed continuously increasing the range of problems that can be studied. In this review, the methodology of the family of HDNNPs including new recent developments will be discussed using a classification scheme into four generations of potentials, which is also applicable to many other types of ML potentials. The first generation is formed by early neural network potentials designed for low-dimensional systems. High-dimensional neural network potentials established the second generation and are based on three key steps: first, the expression of the total energy as a sum of environment-dependent atomic energy contributions; second, the description of the atomic environments by atom-centered symmetry functions as descriptors fulfilling the requirements of rotational, translational, and permutation invariance; and third, the iterative construction of the reference electronic structure data sets by active learning. In third-generation HDNNPs, in addition, long-range interactions are included employing environment-dependent partial charges expressed by atomic neural networks. In fourth-generation HDNNPs, which are just emerging, in addition, nonlocal phenomena such as long-range charge transfer can be included. The applicability and remaining limitations of HDNNPs are discussed along with an outlook at possible future developments.

From Molecular Fragments to the Bulk: Development of a Neural Network Potential for MOF-5.

The development of first-principles-quality reactive atomistic potentials for organic-inorganic hybrid materials is still a substantial challenge because of the very different physics of the atomic interactions-from covalent via ionic bonding to dispersion-that have to be described in an accurate and balanced way. In this work we used a prototypical metal-organic framework, MOF-5, as a benchmark case to investigate the applicability of high-dimensional neural network potentials (HDNNPs) to this class of materials. In HDNNPs, which belong to the class of machine learning potentials, the energy is constructed as a sum of environment-dependent atomic energy contributions. We demonstrate that by the use of this approach it is possible to obtain a high-quality potential for the periodic MOF-5 crystal using density functional theory (DFT) reference calculations of small molecular fragments only. The resulting HDNNP, which has a root-mean-square error (RMSE) of 1.6 meV/atom for the energies of molecular fragments not included in the training set, is able to provide the equilibrium lattice constant of the bulk MOF-5 structure with an error of about 0.1% relative to DFT, and also, the negative thermal expansion behavior is accurately predicted. The total energy RMSE of periodic structures that are completely absent in the training set is about 6.5 meV/atom, with errors on the order of 2 meV/atom for energy differences. We show that in contrast to energy differences, achieving a high accuracy for total energies requires careful variation of the stoichiometries of the training structures to avoid energy offsets, as atomic energies are not physical observables. The forces, which have RMSEs of about 94 meV/ a0 for the molecular fragments and 130 meV/ a0 for bulk structures not included in the training set, are insensitive to such offsets. Therefore, forces, which are the relevant properties for molecular dynamics simulations, provide a realistic estimate of the accuracy of atomistic potentials.

A relative energy gradient (REG) study of the planar and perpendicular torsional energy barriers in biphenyl

Biphenyl is a prototype molecule, the study of which is important for a proper understanding of stereo-electronic effects. In the gas phase it has an equilibrium central torsion angle of ~ 45° and shows both a planar (0°) and a perpendicular (90°) torsional energy barrier. The latter is analysed for the first time. We use the newly proposed REG method, which is an exhaustive procedure that automatically ranks atomic energy contributions according to their importance in explaining the energy profile of a total system. Here, the REG method operates on energy contributions computed by the interacting quantum atoms method. This method is minimal in architecture and provides a crisp picture of well-defined and well-separated electrostatic, steric and exchange (covalent) energies at atomistic level. It is shown that the bond critical point occurring between the ortho-hydrogens in the planar geometry has been wrongly interpreted as a sign of repulsive interaction. A convenient metaphor of analysing football matches is introduced to clarify the role of a REG analysis.

Energetics of graphene flakes

Based on the idea that the binding energy of a graphene flake is a sum of atomic energy contributions, which depend on the local atomic environment of each atom of the flake, we propose a model for graphene flake energetics, which can very accurately predict the cohesive energy of graphene flakes. In our study, we calculate the cohesive energy of hexagonal graphene flakes with up to ≈1000 atoms using the tight binding molecular dynamics method and we show that the calculated values fit extremely accurately to the expression derived from the proposed model. For a further validation, we show that the proposed expression can very accurately predict the calculated values of other graphene flakes with random shapes. Based on that model, we show that the graphene flake stability obeys the following rules: (1) Between isomers the most stable are those with the larger number of bonds (or equivalently, with the smaller number of edge atoms) and (2) between isomers with the same number of bonds (or with the same number of edge atoms), the most stable are those with the smaller number of zig-zag atoms.

MO-D-BRB-01: Accuracy Requirements and Uncertainties in Radiation Therapy: A Preview of an Upcoming IAEA Report.

The International Atomic Energy Agency (IAEA) has a mandate 'to accelerate and enlarge the contribution of atomic energy to peace, health and prosperity throughout the world'. Through its Human Health Division, this is done by developing guidance documents on standards of practice in radiation medicine. In recent years, there have been major advances in the technology of radiation oncology which have allowed for a transition from conventional 2-D radiation therapy to the implementation of 3-D conformal radiation therapy (CRT), intensity modulated radiation therapy (IMRT), image-guided radiation therapy (IGRT), adaptive radiation therapy (ART), and 4-D imaging and motion management in radiation therapy. Brachytherapy procedures have also evolved both for high dose rate (HDR) techniques as well as permanent implants, especially for prostate cancer treatments. Multiple imaging modalities are now available for target volume and normal tissue delineation for radiation treatment planning both for external beam radiation therapy and brachytherapy. With these new advanced technologies and improved outcome considerations for both external beam and brachytherapy, there is also a recognized need for greater accuracy in the radiation treatment process. While a number of reports and publications have defined accuracy needs in radiation oncology, most of these reports were developed in an era with different radiation technologies and date back to the 1980s and 90s. In view of the new technologies and techniques, improvements in dosimetry methodologies and new clinical dose- volume data, the IAEA is developing a new international guidance document on 'Accuracy Requirements and Uncertainties in Radiation Therapy' in order to reduce these uncertainties to provide more effective and safer patient treatments. This review will summarize the new IAEA report which is expected to be published later in 2012. This work is supported by the International Atomic Energy Agency.

Construction of high-dimensional neural network potentials using environment-dependent atom pairs

An accurate determination of the potential energy is the crucial step in computer simulations of chemical processes, but using electronic structure methods on-the-fly in molecular dynamics (MD) is computationally too demanding for many systems. Constructing more efficient interatomic potentials becomes intricate with increasing dimensionality of the potential-energy surface (PES), and for numerous systems the accuracy that can be achieved is still not satisfying and far from the reliability of first-principles calculations. Feed-forward neural networks (NNs) have a very flexible functional form, and in recent years they have been shown to be an accurate tool to construct efficient PESs. High-dimensional NN potentials based on environment-dependent atomic energy contributions have been presented for a number of materials. Still, these potentials may be improved by a more detailed structural description, e.g., in form of atom pairs, which directly reflect the atomic interactions and take the chemical environment into account. We present an implementation of an NN method based on atom pairs, and its accuracy and performance are compared to the atom-based NN approach using two very different systems, the methanol molecule and metallic copper. We find that both types of NN potentials provide an excellent description of both PESs, with the pair-based method yielding a slightly higher accuracy making it a competitive alternative for addressing complex systems in MD simulations.

The International Atomic Energy Agency’s Activities in Radiation Medicine and Cancer: Promoting Global Health Through Diplomacy

Global health has been an issue of seemingly low political importance in comparison with issues that have direct bearing on countries’ national security. Recently, health has experienced a “political revolution” or a rise in political importance. Today, we face substantial global health challenges, from the spread of infectious disease, gaps in basic maternal and child health care, to the globalization of cancer. A recent estimate states that the “overall lifetime risk of developing cancer (both sexes) is expected to rise from more than one in three to one in two by 2015.” These issues pose significant threats to international health security. To successfully combat these grave challenges, the international community must embrace and engage in global health diplomacy, defined by scholars Thomas Novotny and Vicanne Adams as a political activity aimed at improving global health, while at the same time maintaining and strengthening international relations. The IAEA (International Atomic Energy Agency) is an international organization with a unique mandate to “accelerate and enlarge the contribution of atomic energy to peace, health, and prosperity throughout the world.” This article discusses global health diplomacy, reviews the IAEA’s program activities in human health by focusing on radiation medicine and cancer, and the peaceful applications of atomic energy within the context of global health diplomacy.

A neural network potential-energy surface for the water dimer based on environment-dependent atomic energies and charges

Understanding the unique properties of water still represents a significant challenge for theory and experiment. Computer simulations by molecular dynamics require a reliable description of the atomic interactions, and in recent decades countless water potentials have been reported in the literature. Still, most of these potentials contain significant approximations, for instance a frozen internal structure of the individual water monomers. Artificial neural networks (NNs) offer a promising way for the construction of very accurate potential-energy surfaces taking all degrees of freedom explicitly into account. These potentials are based on electronic structure calculations for representative configurations, which are then interpolated to a continuous energy surface that can be evaluated many orders of magnitude faster. We present a full-dimensional NN potential for the water dimer as a first step towards the construction of a NN potential for liquid water. This many-body potential is based on environment-dependent atomic energy contributions, and long-range electrostatic interactions are incorporated employing environment-dependent atomic charges. We show that the potential and derived properties like vibrational frequencies are in excellent agreement with the underlying reference density-functional theory calculations.

Extension of energy density analysis to periodic-boundary-condition calculations with plane-wave basis functions

Energy density analysis (EDA), which partitions the total energy of a molecular system into atomic contributions, has been extended to periodic-boundary-condition calculations carried out by density-functional theory using plane-wave (PW) basis functions and pseudopotentials. The present analysis method, called PW-EDA, allows us to extract local-energy information by estimating atomic energy contributions. Numerical applications to an isolated molecule ${\text{H}}_{2}\text{O}$ and the adsorption of ${\text{C}}_{2}{\text{H}}_{2}$ on the $\text{Si}(001)\text{\ensuremath{-}}(2\ifmmode\times\else\texttimes\fi{}1)$ surface confirm the reliability and usefulness of PW-EDA.

Grid-based energy density analysis: Implementation and assessment

Grid-based energy density analysis (grid-EDA) that decomposes the total energy into atomic energies by a space-partitioning function is proposed. The kinetic energy, nuclear attraction, and exchange-correlation functional are evaluated on grid points and are split into atomic contributions. To reduce numerical errors in the conventional scheme of numerical integration, the electronic Coulomb and HF exchange interactions are evaluated by the pseudospectral method, which was first applied to an ab initio method by Friesner [Chem. Phys. Lett. 116, 39 (1985)], and are decomposed into atomic contributions. Grid-EDA using the pseudospectral method succeeds in ensuring less than 1 kcalmol error in total energies for small molecules and providing reliable atomic energy contributions for the problematic lithium cluster, which exhibits a strong basis-set dependence for Mulliken-type EDA. Also, site-dependent atomization energies are estimated by grid-EDA for cluster models such as Li(48), C(41)H(60), and Mg(32)O(32). Grid-EDA reveals that these models imitate crystal environments reasonably because atomization energies estimated from the inner atoms of the models are close to the experimental cohesive energies.

IAEA's role in the global management of cancer-focus on upgrading radiotherapy services

The International Atomic Energy Agency (IAEA) is an intergovernmental organization composed by 138 Member States within the United Nations. It has a mandate to seek to accelerate and enlarge the contribution of atomic energy to peace, health and prosperity throughout the world. Within the IAEA structure, the Division of Human Health contributes to the enhancement of the capabilities in Member States to address needs related to prevention, diagnosis and treatment of health problems through the development and application of nuclear and radiation techniques within a framework of quality assurance. In view of the increasing cancer incidence rates in developing countries the activities in improving management of cancer have become increasingly important. This review will outline the IAEA's role in cancer management focusing on activities related to improving radiotherapy worldwide.

The American Chemical Society's Summer Schools in Nuclear and Radiochemistry

Uses of radioisotopes and radiation in medicine, industry, agriculture are amongst the most beneficial applications of atomic energy for peaceful purposes. The International Atomic Energy Agency aims - in accordance with its statute - to accelerate and enlarge the contribution of atomic energy to peace, health and prosperity throughout the world. The various Technical Divisions of the Agency carry out this mandate and enhance, particularly through 'manpower development activities' the nuclear applications of radiation and isotopes. Staff members of the 'Industrial Applications and Chemistry Section' in the Department of Nuclear Sciences and Applications of the IAEA have supported more than 600 trainees per year for education in nuclear and radiochemistry through fellowships, training courses and workshops. Provision of printed material in various disciplines is supporting our educational efforts.

Analytical Methods: Improvements, Advancements and New Horizons

The workshop and exhibits dealing with analytical methods were selected to highlight the current state of the art in elemental analysis. The presentations in the first part of the workshop described approaches and advances important to the analysis of trace minerals. These presentations included: 1) two approaches to mass spectrometry, inductively coupled plasma and accelerator mass spectrometry; 2) use of nuclear magnetic resonance in studies of mineral function; and 3) the use and limitations of fluorescent probes in studies of metal uptake and regulation. In the second part of the workshop, the International Atomic Energy's contributions to nutritional "metrology" were described. Advances in instrumentation over the past decade have led to extraordinary improvements in the precision and sensitivity of mineral analyses. The ability to address isotopic speciation at such low levels sets the stage for numerous novel approaches in the assessment of trace mineral function.

Economic prerequisites for the development of nuclear power

The development of nuclear power, as no other field of human endeavor, has revealed the need for predicting the consequences of nuclear power not only in the production of energy itself, but also in the ecology, economics, and even politics. On the one hand, the future of nuclear power is determined by a society`s attitude toward nuclear power and depends on economic possibilities. On the other hand, the future society and the economic situation that will develop in the world will largely depend on the amount of energy accessible to mankind and the method used to obtain it, and therefore also the relative contribution of atomic energy to the total balance of energy production. In declaring its attitude toward nuclear power, society is now determining to a definite extent not only the future of nuclear power but also nuclear power itself. This article is an abstract of the entire report.

Electronic Structures of Linear Polymers. II. Formulation and CNDO/2 Calculation for Polyethylene and Poly(tetrafluoroethylene)

The Hartree–Fock procedure for infinite linear polymers is formulated by the use of the cyclic boundary condition. An actual calculation using this procedure is carried out for the various conformations of infinite polyethylene and poly(tetrafluoroethylene) in the semiempirical CNDO/2 SCF approximation. Polyethylene has an energy minimum at the C–C rotational angle ω = 180° or for the trans zigzag conformation, while poly (tetrafluoroethylene) has a minimum around ω = 162°, a slightly twisted trans zigzag conformation. Both are in excellent agreement with experimental results for the single crystals. Both polymers, experimentally good electric insulators, are calculated to have a large energy gap between the highest-filled and the lowest-vacant band. The segmental and atomic energy contributions and charge distributions are discussed in detail in comparison with alkanes and as functions of the conformational angle ω.

Pandora's Nuclear Box

The most dramatic aspect of the nuclear age has been the strategic arms race that it has inspired. For nearly a decade following Hiroshima, questions of the development and control of nuclear arms dominated discussion and debate in the high political councils of the world's major powers, and the race for leadership in military nuclear capability was the only aspect of the nuclear phenomenon that captured public attention and scholarly analysis. While the nuclear arms debate continues unabated, as evidenced by the recently concluded Non-Proliferation Treaty (npt) and the current strategic arms limitation talks (salt) between Washington and Moscow, broader nuclear vistas have opened up. President Eisenhower's Atoms for Peace proposal of 1953, followed by the 1955 Geneva Conference on the Peaceful Uses of Atomic Energy, and the creation in 1957 of the International Atomic Energy Agency (iaba) accelerate and enlarge the contribution of atomic energy to peace, health and prosperity throughout the world impressed on the public consciousness an awareness that the atom had a benign and constructive dimension as well as destructive capabilities. In 1970 this awareness is much more manifest, and a significantly larger portion of time and energy devoted to nuclear questions is being invested in the peaceful nuclear domain. The prevailing, though not unchallenged attitude among Elites and publics in advanced and developing nations is one of anticipation of the potential benefits to be derived from the harnessing of the peaceful atom. But this quasieuphoric view, as the books by Novick* and by Curtis and Hogan2 reveal, is built on a combination of grossly inadequate public information and knowledge regarding the risks inherent in the industrialization of atomic power, and high-level, high-powered salesmanship by spokesmen of government and private industry. The message of these two studies on the peaceful atom is that apprehensiveness rather than anticipation would be a more appropriate mood; that nuclear power development is being exploited at a pace that outdistances our command of the technology; and that the longterm consequences of the continued commercialization of the atom at the present rate and under existing conditions are at best risky and at worst potentially disastrous. The development of the power reactor programme in the United States dates from the mid-1950s, when the Atomic Energy Commission, supported by industries which had heavily invested in military nuclear technology and by proponents of the private ownership of utilities, ended the government monopoly in the nuclear