simulointi symmetria grafeeni hiilinanoputket kimmoisuus nanorakenteet taipuvuus venyminen Bloch's Theorem Gaussian curvature RPBC atomic simulation environment (ASE) bending carbon nanotubes density-functional theory (DFT) density-functional tight-binding (DFTB) elasticity theory graphene nanoputket nanoscience nanotieteet revised periodic boundary conditions simulation cell stretching symmetry twisting