+-- Dr. Andrei Postnikov ---- Tel. +33-387315873 ----- mobile +33-666784053 ---+ | Paul Verlaine University - Institute de Physique Electronique et Chimie, | | Laboratoire de Physique des Milieux Denses, 1 Bd Arago, F-57078 Metz, France | +-- [EMAIL PROTECTED] ------------ http://www.home.uni-osnabrueck.de/apostnik/ --+
On Tue, 5 Dec 2006, siesta88 wrote: | Dear SIESTA users: | I used dzp atomic basis,160 ry meshcutoff ,30 Bohr kgrid cutoff to calculate FePt system(see the pictures of my unit cell in attachment), however, it seems to take forever to converge with the SCF cycle. dDMax(around 1.0) stays far beyond tolerence (0.00001), and if I increase the cell to 108 atom unit cell(3x3x3 supercell), dDmax becomes even larger. Yet the simplest unit cell with 4 atoms converges fine. | | Here is a fragment of my SCF cycle: | siesta: 42 -25183.5213 -24873.4575 -24873.4883 1.2337 -2.1597 | siesta: 43 -25153.3555 -24872.5874 -24872.6213 1.0953 -1.7754 | siesta: 44 -25204.6434 -24870.7750 -24870.8201 1.0794 -2.1741 | siesta: 45 -25174.8238 -24868.9171 -24868.9419 1.3392 -2.3371 | siesta: 46 -25138.7377 -24868.4595 -24868.4879 1.0200 -2.1080 | siesta: 47 -25177.5378 -24866.3887 -24866.4199 1.1245 -1.9894 | siesta: 48 -25159.6251 -24865.8291 -24865.8733 1.0482 -2.0247 | siesta: 49 -25206.9572 -24864.1873 -24864.2264 1.0436 -2.1378 Dear SuiYang: Your mixing parameter is too large redata: New DM Mixing Weight = 0.2500 that's why (at least) you get no convergence. Try to set DM.MixingWeight .05 (or less), to begin with. There are two important issues to consider: 1) in a large supercell, there is genererally more difficult to achieve convergence because you have many bands with contributions from similar atoms around the Fermi level. So the system has a tendency to swap charge from one atom to another, which effect you might need to damp more eficiently than was the case in a smaller cell. In the single cell with only few bands and few atoms, you won't normally have this "degree of freedom" in your system, so the convergency goes sraightforwardly. 2) In a metal system, depending on the actual band sructure, the accuracy of the k-summation/interation becomes an important issue influencing the accuracy of your result, and the convergence to it. In SIESTA where no tetrahedra integration is available, you must carefully test the effect of increasing number of k-points, and the energy broadening used in the search for the Fermi level. Your present choice 5*5*5 k-points with ElectronicTemperature 300 K is not a priori unreasonable for getting a preliminary convergence, but be careful to make additional tests if you want to extract some energy differences afterwards. Best regards, Andrei Postnikov +-- Dr. Andrei Postnikov ---- Tel. +33-387315873 ----- mobile +33-666784053 ---+ | Paul Verlaine University - Institute de Physique Electronique et Chimie, | | Laboratoire de Physique des Milieux Denses, 1 Bd Arago, F-57078 Metz, France | +-- [EMAIL PROTECTED] ------------ http://www.home.uni-osnabrueck.de/apostnik/ --+
