• Atomic and molecular physics is a field of physics that studies the structure and properties of atoms and molecules. (researchandmarkets.com)
  • Atomic and molecular physics focuses on the behavior of electrons and nuclei in atoms and molecules, and the interactions between them. (researchandmarkets.com)
  • It also studies the structure and properties of matter at the atomic and molecular level, and the interactions between atoms and molecules. (researchandmarkets.com)
  • The application of quantum theory to atomic and molecular structure, and the interaction between electromagnetic radiation and atoms and simple molecules. (uoguelph.ca)
  • Molecular orbitals and qualitative treatment of H2 and first row diatomic molecules. (uoguelph.ca)
  • The aim of computational molecular design is the identification of promising hypothetical molecules with a predefined set of desired properties. (springer.com)
  • One major difficulty in the computational creation of molecules is the exclusion of the occurrence of chemically unfavorable structures. (springer.com)
  • Computational molecular design has a great potential to promote enormous savings in time and cost in the discovery and development of functional molecules and assembles including drugs, dyes, solvents, polymers, and catalysis. (springer.com)
  • Quantitation of adsorbed lipids by LC-MS along with the structural assessments of phospholipid binding by atomic force microscopy and molecular modeling indicated that the phospholipids (approximately 108 molecules per SWCNT) formed an uninterrupted "coating" whereby the hydrophobic alkyl chains of the phospholipids were adsorbed onto the SWCNT with the polar head groups pointed away from the SWCNT into the aqueous phase. (cdc.gov)
  • There were many challenges to overcome in designing and implementing QMPFF3-based molecular dynamics simulations especially to study the behavior of water molecules next to hydrophobic surfaces at atomic detail and subpicosecond time resolution. (phys.org)
  • Thanks to these properties, Rydberg atoms form a bridge between the microscopic world (atoms, electrons, and molecules) and the mesoscopic world of micrometer length scales, optical lattices, Bose-Einstein condensates, and structured or amorphous condensed matter. (mpg.de)
  • We are interested in exploring theoretically the structure and dynamics of Rydberg matter -- encompassing intera interacting Rydberg atoms, ground state atoms immersed within the electronic wave function of a Rydberg atom, Rydberg molecules, and Rydberg quasiparticles (excitons, polarons, etc). (mpg.de)
  • likewise if it freezes to become solid ice, nothing has changed with regard to the basic chemical structure of the H 2 O molecules that make up water. (encyclopedia.com)
  • X-ray crystallography yields atomic-resolution 3D images of the whole spectrum of molecules ranging from small inorganic clusters to large protein complexes constituting the macromolecular machinery of life. (europa.eu)
  • Overview of atomic and molecular structure implying a specialisation especially regarding molecules. (lu.se)
  • The structure of mixed methanol/chloroform clusters from core-level photoelectron spectroscopy and modeling. (uib.no)
  • Valence photoelectron spectroscopy of N-2 and CO: Recoil-induced rotational excitation, relative intensities, and atomic orbital composition of molecular orbitals. (uib.no)
  • Welcome to the 2019 course in Atomic & molecular spectroscopy! (lu.se)
  • The aim of the course is to provide theoretical and practical knowledge on the many powerful methods provided by modern atomic- and molecular spectroscopy regarding basic studies as well as practical applications. (lu.se)
  • The course intends to provide theoretical and practical knowledge of the many powerful methods, as modern atom and molecular spectroscopy offer respect both basic studies and practical applications. (lu.se)
  • The aim of the course is to provide knowledge in modern atom and molecular spectroscopy with special emphasis on practical applications. (lu.se)
  • The work marks the first such structure determined using the university's $17 million electron cryo-microscopy (cryo-EM) facility that opened last year. (cam.ac.uk)
  • Frank Bürmann, a member of Jan's group and the study's lead author, purified MukBEF and assembled a stable complex bound to MatP to resolve the complex structure using electron cryomicroscopy (cryo-EM). (cam.ac.uk)
  • Here we report the cryo-electron microscopy structure of a tubular HIV-1 capsid-protein assembly at 8 Å resolution and the three-dimensional structure of a native HIV-1 core by cryo-electron tomography. (nature.com)
  • The cryo-electron-microscopy structures enable modelling by large-scale molecular dynamics simulation, resulting in all-atom models for the hexamer-of-hexamer and pentamer-of-hexamer elements as well as for the entire capsid. (nature.com)
  • In atomic theory and quantum mechanics, an atomic orbital (/ˈɔːrbɪtəl/) is a function describing the location and wave-like behavior of an electron in an atom. (wikipedia.org)
  • The term atomic orbital may also refer to the physical region or space where the electron can be calculated to be present, as predicted by the particular mathematical form of the orbital. (wikipedia.org)
  • Atomic orbitals are the basic building blocks of the atomic orbital model (or electron cloud or wave mechanics model), a modern framework for visualizing the submicroscopic behavior of electrons in matter. (wikipedia.org)
  • In this model the electron cloud of an atom may be seen as being built up (in approximation) in an electron configuration that is a product of simpler hydrogen-like atomic orbitals. (wikipedia.org)
  • Atomic orbitals exactly describe the shape of this "atmosphere" only when one electron is present. (wikipedia.org)
  • Specifically, in quantum mechanics, the state of an atom, i.e., an eigenstate of the atomic Hamiltonian, is approximated by an expansion (see configuration interaction expansion and basis set) into linear combinations of anti-symmetrized products (Slater determinants) of one-electron functions. (wikipedia.org)
  • Parallel systems, which bear interesting similarities and connections to Rydberg matter, include highly correlated atomic negative ions, quantum dots, electron-hole pairs, and more. (mpg.de)
  • We will apply our new capabilities to one of the most important problems in structural biology, which is to elucidate the dynamics of light reactions, electron transfer and protein structure in photosynthesis. (europa.eu)
  • Photoionization of an atomic electron is followed by electronic relaxation which results in a migration of charge within the atom. (lu.se)
  • Quantum control of HHG is a vast subject with applications from the microscopic atomic scale, where electron correlation and molecular dynamics, to the macroscopic scale of optics. (lu.se)
  • Now, Jan Löwe's group in the LMB's Structural Studies Division, assisted by Jason Chin's group in the PNAC Division, have determined the most complete structure of an SMC complex to date. (cam.ac.uk)
  • Cryo-EM structural data have been deposited in the EMDataBank under accession codesEMD-5582 andEMD-5639, and the MDFF atomic model of the CA HOH and models of HIV-1 capsid have been deposited in the Protein Data Bank under accession numbers 3J34 , 3J3Q , 3J3Y . (nature.com)
  • In this study, molecular dynamics simulations were conducted to investigate the effects of contact area and structural stiffness of corrugated nano-structures on the fundamental frictional behavior at the atomic-scale. (elsevierpure.com)
  • Using ideas from computer vision, we adapt a specific type of neural network called a PointNet to identify local structural environments in molecular simulations. (osti.gov)
  • With the help of the Titan supercomputer at the Oak Ridge Leadership Computing Facility in Tennessee and the Blue Waters supercomputer at the National Center for Supercomputing Applications (NCSA) at Illinois, the researchers were the first to provide the atomic-level structural model of the immature retroviral lattice of RSV. (sciencedaily.com)
  • Dr. Gaurav Chopra and Professor Michael Levitt in the Department of Structural Biology used molecular dynamics simulations involving a state-of-the-art Quantum Mechanical Polarizable Force Field (QMPFF3) to study the hydration of buckminsterfullerene, the smallest hydrophobic nanosphere widely referred to as a buckyball or C 60 . (phys.org)
  • LRRs are composed of consecutively arranged structural units (repeats) that form elongated structure. (lu.se)
  • To explore the possibility of the c-erbB2 oncogene antisense probe labeled with superparamagnetic iron oxide (SPIO) nanoparticles as a target contrast agent for magnetic resonance (MR) imaging whose morphology was observed with atomic force microscopy (AFM), and its efficiency was examined by MR imaging. (molvis.org)
  • With atomic force microscopy (AFM), we observed the morphology of antisense probes labeled with SPIO to explore its potential as a target contrast agent for magnetic resonance (MR) imaging. (molvis.org)
  • My project is about using the scanning probe microscopy to investigate atomic structures and molecular assemblies on the superconducting material. (eucor-uni.org)
  • Optical super-resolution microscopy of molecular mechanisms of disease. (lu.se)
  • It is a subfield of atomic, molecular, and optical physics, and is closely related to other fields such as quantum chemistry, condensed matter physics, and biophysics. (researchandmarkets.com)
  • Atomic and molecular physics is used in a variety of industries, including energy, healthcare, and materials science. (researchandmarkets.com)
  • Companies in the atomic and molecular physics market include Thermo Fisher Scientific, Oxford Instruments, Bruker Corporation, JEOL Ltd., and Hitachi High-Technologies Corporation. (researchandmarkets.com)
  • Erwin Schrödinger's idea that physics could help solve biological riddles was the spark that led many researchers to try to unlock the secrets behind our book of life, the structure of DNA. (nobelprize.org)
  • Physical Review A. Atomic, Molecular, and Optical Physics (PRA). (uib.no)
  • Journal of Physics B: Atomic, Molecular and Optical Physics. (uib.no)
  • The course covers theoretical models for the structure and reactions of atomic nuclear, as well as experiments in nuclear physics and their scientific applications. (lu.se)
  • account for spectroscopic methods in different energy intervals · describe the most common components in spectroscopic equipment for different energy intervals with special emphasis on optical methods and in particular laser spectroscopic methods · describe what can be measured using spectroscopic techniques · describe a consolidated image of quantum mechanics and atomic physics and its relation to classical physics. (lu.se)
  • In 2004, after her degree project in atomic physics, Sara Thorin was granted a doctoral studentship at MAX Lab, a workplace she describes as a haven for physicists and engineers. (lu.se)
  • We present 3D high resolution numerical simulations of turbulent converging interstellar atomic hydrogen flows. (eas-journal.org)
  • First, we report pure hydrodynamical simulations from which we infer statistics of the CNM structures. (eas-journal.org)
  • Identifying local structure in molecular simulations is of utmost importance. (osti.gov)
  • In contrast, our approach is a generic framework that requires no system-specific feature engineering and operates on the raw output of the simulations, i.e., atomic positions. (osti.gov)
  • Our results suggest the approach will be broadly applicable to many types of local structure in simulations. (osti.gov)
  • article{osti_1542673, title = {A generalized deep learning approach for local structure identification in molecular simulations}, author = {DeFever, Ryan S. and Targonski, Colin and Hall, Steven W. and Smith, Melissa C. and Sarupria, Sapna}, abstractNote = {Identifying local structure in molecular simulations is of utmost importance. (osti.gov)
  • Having a precise description of water s structure is critical to constructing accurate simulations of molecular events, including protein folding, substrate binding, macromolecular recognition, and complex formation. (phys.org)
  • A more detailed, atomic-level description of the structure, dynamics and compaction of IDPs, may however, be obtained from molecular simulations, but these need to be validated or generated by comparison with experiments. (lu.se)
  • The nano-structured surface was modeled as an array of corrugated carbon atoms with a given periodicity. (elsevierpure.com)
  • The most popular method to study the water structure around any solute used a one-dimensional radial distribution function that averages the number of water O and H atoms at a particular distance to increase the signal to noise ratio, Chopra explains. (phys.org)
  • Curiously, the interaction between Rydberg atoms can even lead to additional crystalline structure via the Rydberg blockade. (mpg.de)
  • Weakly bound, atomic and molecular, clusters have been the subject of many studies to investigate structures, dynamics and energetics of nano-scale objects, with the goal to understand the evolution of their properties from isolated atoms to the bulk solid. (lu.se)
  • Collisional Effects on Molecular Spectra. (researchandmarkets.com)
  • Interpretation of molecular spectra, deduction of molecular constants. (uoguelph.ca)
  • Gerhard Herzberg's "Atomic spectra and atomic structure" and "Molecular spectra and molecular structure, Vol 1" are gold mines of experimental information with wonderful qualitative discussions. (uoguelph.ca)
  • Molecular Spectra As a Tool in Assigning Carbon 1s Photoelectron Spectra of Physisorbed Overlayers. (uib.no)
  • 9th International Colloquium on Atomic Spectra and Oscillator Strengths (ASOS9). (lu.se)
  • Atomic orbitals may be defined more precisely in formal quantum mechanical language. (wikipedia.org)
  • Atomic model for the MukBEF-DNA complex (left) and in vivo loop binding as determined by chemical circularisation of the protein (right). (cam.ac.uk)
  • Structure of the amino-terminal core domain of the HIV-1 capsid protein. (nature.com)
  • Crystal structure of dimeric HIV-1 capsid protein. (nature.com)
  • Structure of the HIV-1 full-length capsid protein in a conformationally trapped unassembled state induced by small-molecule binding. (nature.com)
  • It can also be used to observe protein structure in dynamics and solution, and to determine the averaged particle size, shape, and distribution, as well as surface-to-volume ratio. (lu.se)
  • The atomic interactions between subunits in protein assemblies are finely tuned to control affinity as well as quaternary structure. (lu.se)
  • Revealing how molecular interactions are optimized to control both oligomerization state and assembly pathway in protein assemblies is of vital importance for our understanding of biomedically important biological assemblies, as well as for efforts to manipulate them. (lu.se)
  • Our goal is to achieve molecular level control of protein self-assembly. (lu.se)
  • We use structure based modeling and computational protein design methods to optimize the protein sequence of proteins and peptides in order to control their assembly. (lu.se)
  • This perspective focuses on two areas that have yielded new useful information during the last 20 years: (i) structure-activity relationship (SAR) studies of contact allergy based on the concept of hapten-protein binding and (ii) mechanistic investigations regarding activation of nonsensitizing compounds to contact allergens by air oxidation or skin metabolism. (cdc.gov)
  • Protein structures are invaluable for biochemical studies and for mechanical or molecular mechanics calculations have become the understanding of how proteins work. (lu.se)
  • The most common strong alternative and complement to experimental methods to methods to obtain protein structures are X-ray crystallography obtain insight in biochemical processes. (lu.se)
  • For metal- powerful methods to obtain and compare reaction and activation loproteins, a third method to obtain local information about the energies for suggested enzyme mechanisms and they can also provide atomic details about the protein dynamics. (lu.se)
  • In this regard, the frictional behavior may be controlled by varying the surface morphology through nano-structuring. (elsevierpure.com)
  • Furthermore, stiffness of the nano-structure was identified to be an important variable in dictating the frictional behavior. (elsevierpure.com)
  • Kim, HJ & Kim, DE 2012, ' Molecular dynamics simulation of atomic-scale frictional behavior of corrugated nano-structured surfaces ', Nanoscale , vol. 4, no. 13, pp. 3937-3944. (elsevierpure.com)
  • Yet physical and chemical changes have at least one thing in common: they can be explained in terms of behavior at the atomic or molecular level. (encyclopedia.com)
  • Hopfner, K. P. & Tainer, J. A. Rad50/SMC proteins and ABC transporters: unifying concepts from high-resolution structures. (nature.com)
  • Structure determination using X-ray crystallography is often used in drug discovery to proteins and their active sites at atomic resolution. (lu.se)
  • Expert systems containing information about the relationship between the chemical structure and the ability of chemicals to haptenate proteins are available. (cdc.gov)
  • In spite of being the first testing ground of Quantum Mechanics, almost hundred years ago, Atomic Structure Theory is a vibrant and active area. (lu.se)
  • The structure of the tubular assembly shows, at the three-fold interface 11 , a three-helix bundle with critical hydrophobic interactions. (nature.com)
  • this has been under-represented by the limitations due to approximate modeling of atomic interactions in the empirical force fields widely used for the past decades. (phys.org)
  • Paper V contains an investigation of the effects on shell structure in fermionic droplets with dipole-dipole interactions. (lu.se)
  • Our investigations are aimed at understanding how molecular interactions are tuned in coiled coils to encode specific oligomerization states and how at the same time others are selected against. (lu.se)
  • The local structure of small water clusters: imprints on the core-level photoelectron spectrum. (uib.no)
  • The team has extended it to include characterization of the distribution of molecular clusters in fluids with important energy, chemical, and biological applications. (newswise.com)
  • Using molecular modeling and large-scale molecular dynamic simulation, University of Illinois researchers constructed an atomic model of an immature retrovirus. (sciencedaily.com)
  • This profusely illustrated book by a world-renowned chemist and award-winning chemistry teacher provides a complete introduction to atomic and molecular structure and bonding for science students. (aip.org)
  • Awarded to the student achieving the highest overall grade in CHEM 282: Atomic and Molecular Structure. (ualberta.ca)
  • The eukaryotic SMC complexes cohesin and condensin are thought to fold interphase and mitotic chromosomes, respectively, into large loop domains, although the underlying molecular mechanisms have remained unknown. (nature.com)
  • With that knowledge we can try to understand the precise molecular mechanisms of virus maturation and help to elucidate how drugs can be designed to interfere with that. (sciencedaily.com)
  • Through the manipulation of materials at atomic, molecular, and macromolecular levels, nanotechnology makes it possible to create new structures and systems with unique properties and benefits. (cdc.gov)
  • Such gases are routinely studied in the laboratory and can be either completely randomly arranged, as in a thermal vapor, or structured, as in a tweezer array or optical lattice. (mpg.de)
  • The use of molecular mechanics calculations to supplement experimental data in standard X-ray crystallography and NMR refinements is discussed and it is shown that structures can be locally improved by the use of quantum chemical calculations. (lu.se)
  • Because so much information had to be packed into every cell, it must be packed into what Schrödinger called a 'hereditary code-script' embedded in the molecular fabric of chromosomes. (nobelprize.org)
  • While the former design process is referred to as the quantitative structure-property relationship (QSPR) analysis, the latter is known as the inverse-QSPR analysis [ 1 - 9 ]. (springer.com)
  • UT Southwestern Medical Center researchers have published a 3-D atomic structure of the ion channel found in mammals that is implicated in a rare, inherited neurodegenerative disease in humans. (cam.ac.uk)
  • Now an atomic model of the immature retrovirus RSV has been constructed by researchers in order to understand and block the virus. (sciencedaily.com)
  • The researchers, from the Theoretical and Computational Biophysics Group at the Beckman Institute for Advanced Science and Technology at Illinois, published their work in the journal Structure . (sciencedaily.com)
  • An important step forward in creating such a description has been demonstrated at Stanford University School of Medicine, where researchers discovered that polarization increases ordered water structure. (phys.org)
  • The LumoMat-E project (LUmière MOlécule MATériaux) aims to use the tools of chemistry to initiate unprecedented developments in the field of molecular materials for organic electronics and photonics. (univ-angers.fr)
  • This technique will revolutionize our understanding of structure and function at the atomic and molecular level and thereby unravel fundamental processes in chemistry and biology. (europa.eu)
  • Multiconfigurational Quantum Chemistry provides a timely and in-depth overview of this modern methodology that enhances our understanding of atomic states and structures in molecular systems. (lu.se)
  • It comprises four technical facilities on a single site and provides services to companies and research laboratories (molecular design, physico-chemical characterization, device development, etc. (univ-angers.fr)
  • First, cold molecular gas falling towards the black hole forms a disk near the plane of rotation. (sciencedaily.com)
  • This dissertation deals with finite-size effects in a few different quantum many-body phenomena in ultracold atomic systems. (lu.se)
  • Overview of atomic structure: Interaction and energy scales, qualitative effects of spin, Pauli principle. (uoguelph.ca)
  • Newswise - Materials science explores how materials behave, including at the atomic and molecular scales. (newswise.com)
  • It is also possible to test the size and shape of the atomic nuclei, to complement results from collisions. (lu.se)
  • One of the world's leading research institutes, our scientists are working to advance understanding of biological processes at the molecular level - providing the knowledge needed to solve key problems in human health. (cam.ac.uk)
  • Unfortunately, the complexity and size of the viral particle--an irregular and incomplete hexagonal shell of close to 100 nanometers in size--have prevented the experimental determination of the atomic-level structure of the particle. (sciencedaily.com)
  • 1981. The determination of cadmium, copper, lead and thallium in human liver and kidney tissue by flame atomic absorption spectrometry after enzymatic digestion. (cdc.gov)
  • We then determined the crystal structure of CpNMT bound to Myristoyl-Coenzyme A (MyrCoA) and structures of ternary complexes with MyrCoA and the hit compounds to identify the ligand binding modes. (bvsalud.org)
  • A high-resolution crystal structure of PvNMT bound to a representative selective hybrid compound reveals a unique binding site architecture that includes a selective conformation of a key tyrosine residue. (bvsalud.org)
  • Such calculations can also be used to interpret the structures, e.g. to decide the protonation state of metal-bound ligands. (lu.se)
  • This comparison revealed that the presumptive "donut" is not actually a rigid structure, but instead a complex collection of highly dynamic gaseous components. (sciencedaily.com)
  • This hot atomic gas falls back onto the disk creating a turbulent three dimensional structure. (sciencedaily.com)
  • These features include grain size distributions in metals, the voids and porosity in solid porous materials, and the structures that appear during self- and directed-assembly of soft matter. (newswise.com)
  • 1985. Metals by atomic absorption spectrometry. (cdc.gov)
  • The mol-ecule has an intra-molecular N-H⋯O hydrogen bond between the ammonium cation and the hydro-phosphate anion. (iucr.org)
  • Teaching is primarily delivered at our South Kensington campus, with regular opportunities to advance your skills at our state-of-the-art Molecular Sciences Research Hub in White City. (imperial.ac.uk)
  • Using GAs [ 15 ], which have been more intensively studied, searches for optimal or suboptimal designs by successively modifying chemical structures with genetic operators consisting of mutation, crossover, and selection. (springer.com)
  • It is therefore important to understand in detail the structure and dynamics of these ions, to deduce the property of the plasma, e.g. temperature, density, chemical composition or presence of magnetic fields. (lu.se)
  • I will discuss recent results that provide new insights into how we can compare atomic level structures of IDPs with NMR diffusion, NMR paramagnetic relaxation enhancement and SAXS experiments. (lu.se)
  • A method is called theoretical if it obtains its results from calcula- structure and function of metalloproteins with theoretical methods tions rather than from experiments. (lu.se)
  • Write the atomic symbol for the atom if two protons are added to 18 O. (pearson.com)
  • Low energy atom-atom s-wave scattering and effective (pseudo) potentials with application to atomic traps. (uoguelph.ca)
  • For atomic systems we have arguably a detailed understanding of the forces and we can therefore focus on the many-particle problem. (lu.se)
  • In simple cases order parameters are physically intuitive and trivial to develop (e.g., ion-pair distance), however in most cases, order parameter development becomes a much more difficult endeavor (e.g., crystal structure identification). (osti.gov)
  • We demonstrate the method on crystal structure identification in Lennard-Jones (four different phases), water (eight different phases), and mesophase (six different phases) systems. (osti.gov)
  • The method achieves as high as 99.5% accuracy in crystal structure identification. (osti.gov)
  • This work demonstrates the utility of KTGS to identify small-molecule binders for biological targets where resolved X-ray crystal structures are not readily accessible. (bvsalud.org)
  • They have shown that metal sites in crystal structures are frequently photoreduced or disordered, which makes the interpretation of the structures hard. (lu.se)
  • Topics covered include cell structure and function, DNA replication and the flow of genetic information, enzyme function, metabolism and physiology of microorganisms, plants, and animals. (sfu.ca)
  • Analysis of molecular-based data showed diverse genetic backgrounds among drug-sensitive and MDR TB sensu stricto isolates in contrast to restricted genetic backgrounds among pre-extensively drug-resistant (pre-XDR) TB and XDR TB isolates. (cdc.gov)
  • This new model is the first to give us an atomic-level look at the immature state. (sciencedaily.com)
  • Matter, however, can and does undergo chemical changes, which (as with the various states or phases of matter) are an outcome of activity at the atomic and molecular level. (encyclopedia.com)
  • Created in 2018, the Matrix Federative Research Structure coordinates the Materials department and brings together three research units around a common theme. (univ-angers.fr)
  • We find that the CNM structures are characterized by a mass spectrum d N/ d M ∝ M -1.7-1.8 , a mass-size relation M ∝ L 2.3-2.5 , and a velocity dispersion σ ∝ L 0.4-05 . (eas-journal.org)
  • Of these areas, SAR/quantitative structure-activity relation- ship (QSAR) studies and their implication for contact allergy have been frequently reviewed over the years and will therefore not be scrutinized in detail. (cdc.gov)
  • to obtain structures and their relation to theoretical methods. (lu.se)
  • For soft matter, small-angle scattering is an ideal technique to study structures in the nanometre to micrometre range and get information on the inner structure of disordered or partially ordered materials that are difficult to crystallise. (lu.se)
  • To date, only 1 molecular epidemiologic study has been reported for the Eastern Cape ( 10 ), and it showed that 50% of rifampin-resistant TB isolates (including MDR TB isolates) belonged to the Beijing genotype and that "atypical" Beijing strains were significantly overrepresented. (cdc.gov)