simulointi symmetria Bloch's Theorem atomic simulation environment (ASE) density-functional theory (DFT) density-functional tight-binding (DFTB) grafeeni hiilinanoputket kimmoisuus nanorakenteet nanotieteet simulation cell taipuvuus venyminen 402 Gaussian curvature RPBC bending carbon nanotubes elasticity theory graphene nanoputket nanoscience revised periodic boundary conditions stretching symmetry twisting