Dear SuiYang, > I have used SIESTA to study the property of FePt systems.Here I have a > question about the atomic forces SIESTA calculated: > When I am calculating the FePt unit cell(including 4 atoms) with its > lattice constant around 4 ang.The corresponding atomic forces are around > 0.02 eV/ang.When I am using the 2x2x2 supercell(including 32 atoms) as > unit cell, with relatively the same atomic positions,and the lattice > constant is changed to 8 ang to maintain it is the same bulk as the > previous one,however ,the atomic forces are increased to around one > hundred eV/ang.
at least one problem that you are having concerns the number of k-points. If I understand correctly your FDF files, you're using Gamma point only for both systems. This is a problem for two reasons: 1) your cells are very small and a large number of k-points is possibly needed to be converged; 2) the number of k-points should be consistently scaled when you increase the size of the direct space cell (if you double it along the 3 axis, i.e. if you go from 1x1x1 to 2x2x2, the numer of k-points should be reduced by a factor 2 in the 3 axis as well). Riccardo -- Riccardo Rurali Escola Tècnica Superior d'Enginyeria (ETSE) Universitat Autònoma de Barcelona (UAB) Campus de la UAB 08193 Bellaterra (Barcelona) Spain tel.: +34 93 581 3531 fax.: +34 93 581 2600 e-mail: [EMAIL PROTECTED]
