Density Functional Theory (DFT) Density Functional Theory (DFT) calculations were carried out at T = 0 K with Siesta package 2 and the results were visualized with the Chimera software. We used periodic boundary conditions with vdW-DF2 (or LMKLL) functional that includes van der Waals corrections. The computations were performed with norm-conserving Trouillier-Martins pseudopotentials and double-ζ polarized atomic basis sets. The mesh cut-off used to form the real space grid in the DFT calculations was 300 Ry, and the structural relaxation and geometry optimization were carried out using conjugate-gradient method until the forces and the variation of total energy were less than 0.01 eV/Å and 0.0001 eV, respectively. We considered a vacuum region of 15 Å to minimize the interactions between periodic images in the direction normal to the slab.