NOTICE: The project that is the subject of this report was approved by the Governing Board of the National Research Council, whose members are drawn from the councils of the National Academy of Sciences, the National Academy of Engineering, and the Institute of Medicine. The members of the committee responsible for the report were chosen for their special competences and with regard for appropriate balance.

Support for this project was provided by the National Academy of Sciences, the Howard Hughes Medical Foundation, the American Chemical Society, the U.S. Department of Energy, the National Institute of Standards and Technology, and the Camille and Henry Dreyfus Foundation. Any opinions, findings, conclusions, or recommendations are those of the author(s) and do not necessarily reflect the views of the organizations or agencies that provided support for this project.

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