On Thu, January 24, 2008 12:30, Riccardo Sabatini wrote: > The result i had is around -7.47 Ry. > and there is listed a value of -7.52 Ry for the Si atom free.
What you have to do is break the symmetry of the isolated atom. Silicon atomic configuration is [Ne] 3s2 3p2, nobody knows what it would if it were alone in the universe, but its ground state configuration fills the 3s orbital completely and then, following Hund's rule, put 1 electron each in two p orbitals. Example: 3s 2 3p-x 1 3p-y 1 When you do the pw calculation for the isolated atom symmetry is actively enforced so the occupations will look more like this: 3s 2 3p-x 2/3 3p-y 2/3 3p-z 2/3 In order to break the symmetry you have first to relax its enforcement by specifying nosym=.true. and then you have to break it actively in the initial position by using a non cubic cell, some random starting wfcs or some other similar trick. You can also specify band occupations by hand, e.g. (if you use nbnd=4): occupations = 'from_input' [...] OCCUPATIONS 1. 1. 1. 0. this will suffice in breaking the symmetry. > In the first case i > suppose one has to divide the total energy by the numbers of atoms in > the cell (2 in the first case and 8 in the second) true > but i can't figure out what is energy/cell it's what you get printed on the screen after "Total energy:", in periodic boundary condition you can't compute "Total" energy > Is it correct to define a energy for each atom in a cell in this way > ? (If i remember correctly the difference in energy for the sc > simulation and for the fcc arise from the difference in volume, one is 4 > times than the other not from the numbers of atoms). The sc cell *contains* 4 fcc cells, has 4 times its volume and 4 times its atoms. Its energy per cell is 4 times larger, so the ratio energy/volume must be the same. Also keep in mind that volume for an fcc cell is (alat**3)/4. Please have a look at crystal structure in you favourite solid state book! > caused by the number of plane waves used ? Without seeing you input files I can't know, there may be many other reasons, most likely numbers of k-points (you need 4 times as many k-points in fcc than in sc). bye, LP P.S. please provide your affiliation. -- Lorenzo Paulatto SISSA & DEMOCRITOS (Trieste) +39 040 3787 511 http://people.sissa.it/~paulatto/ ---------------------------------------------------------------- SISSA Webmail https://webmail.sissa.it/ Powered by SquirrelMail http://www.squirrelmail.org/
