Dear SIESTA users, while working with a Silicon surface (7 layers, 2 constrained to bulk geometries (according to previous calculations), Si(100)-(2x1) reconstruction, 2 atoms per layer, with 20.9703 Ang of vacuum and 8.1350 Ang of slab height, BLYP/DZP with BLYP/TM2 PP (tested for transferability and similarity), in a 7×12×1 Monkhorst Pack kGrid and a 3×6×1 supercell, with DM.Tolerance=1.d-5, DM.Energy.Tolerance=1.d-7 eV and MD.MaxForceTol=0.00045 eV/Ang (to coincide with previous B3LYP5/6-311G* and PM7 calculations on non-periodic equivalent systems) and set to PAO.EnergyShift=70 meV and MeshCutoff=200 Ry (as indicated by a total-energy-per-atom convergence analysis)) I ran into two problems: 1) I found 4 vibrational bands between 0 and -200 cm-1; and 2) there is no gap between the 28th and 29th (14 atoms*4 electrons/2 electrons per band=there should be 28 valence bands) electronic bands (or nearby, checked also by plotting the total DOS).
Since the force tolerance is that small, might the imaginary frequecies originate from constraining the lower two layers? In that case, what would you recommend to do next? The fact that the lower layer is not reconstructed could explain the lack of a surface band gap? By capping the lower layer with hydrogen atoms (and thereby transforming the system to some sort of nanoribbon) or working with a symmetric slab (probably a 14 layer system (with the central 2 fixed?)) could the problem be solved? Thanks to all in advance.
