Ab initio density functional theory has been used to analyze flexural modes, elastic constants, and atomic corrugations on single and bi-layer graphene. Frequencies of flexural modes are sensitive to compressive stress; its variation under stress can be related to the anomalous thermal expansion via a simple model based in classical Elasticity Theory [Phys. Rev. B 86, 144103]. Under compression, flexural modes are responsible for a long wavelength rippling with a large amplitude and a marked

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... armonic behavior. This is compared with corrugations created by thermal fluctuations and the adsorption of a light impurity (hydrogen). Typical values for the later are in the sub-Angstrom regime, while maximum corrugations associated to bending modes quickly increase up to a few Angstroms under a compressive stress, due to the intrinsic instability of flexural modes