Graphene layers are stable, hard, and relatively inert. We study how tensile stress affects sigma and pi bonds and the resulting change in the chemical activity. Stress affects more strongly pi bonds that can become chemically active and bind to adsorbed species more strongly. Upon stretch, single C bonds are activated in a geometry mixing 120 and 90 degrees, an intermediate state between sp2 and sp3 bonding. We use ab initio density functional theory to study the adsorption of hydrogen on large clusters and two-dimensional periodic models for graphene. The influence of the exchange-correlation functional on the adsorption energy is discussed.
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