Sorry for my previous mail... I didn't want to send it to everyone.... Sorry again...
> Dear Yuehua, > > it is true that the ecut gives you the number of plane waves used in the > calculation and a test of ecut (fixed lattice, various values of ecut) > gives you the resolution of the energy in your calculation. More plane > waves (larger ecut) lowers the energy of your system. The variation of the > number of plane waves with a change of the volume at fixed ecut was > discussed in PRB 52, 8160 (1995). There, also some corrections to the > total energy with respect to the change of the number of plane waves are > suggested. Of course, if you have a calculation which is converged with > respect to ecut, these errors are very small. > > So, in order to relax your system you first should test the convergence > with respect to ecut (and the number of k) and with these values you > should relax the system with respect to c/a and the volume of the unit > cell. I suggest to relax the c/a for a fixed volume in order to avoid the > spurious variations with respect to the number of plane waves by looking > at Etot(c/a). This you should do for every volume you consider. Having the > volumes with the correct c/a fixed, you can look at Etot(V) in order to > get the relaxed system. > > About the influence of ecutrho I cannot say anything, since it never > played a significant role in my calculations. But maybe someone else could > give further informations. > > Good luck, > Katalin > > > ,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,, > , >>>>>>> Katalin Gaal-Nagy <<<<<<< , > , Phys.Dept./INFM , > , University of Milan , > , Via Celoria 16 , > , +39 02 50317377 , > ,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,, > , >>>>>>> katalin.gaal-nagy at physik.uni-regensburg.de <<<<<<< , > ,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,, > , >>>>>>> Don't dream your life, live your dream <<<<<<< , > ,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,, > > >> thank hande for your advice . >> but i am still puzzled below: >> (1) last time ,i saw a titled " scan the lattice "discussion ,and you could >> find it in forum,they said: >> Basically, as you enalarge your unit cell, your brillouin zone shrinks, >> and at constant cutoff it means that more lattice vectors (i.e. plane >> waves) enter into the fixed cutoff sphere. More plane waves means a >> systematically larger basis set (this is one of the good things >> of plane waves, it's easy to make the basis set more and more complete - >> a nightmare in Gaussian), and, variationally, a lower energy. You see in >> fact your energy drop going to the left. >> >> Check then what is the number of plane waves for each of your >> calculations, but most likely the wfc cutoff is the culprit >> >> so i feel if i keep the encut fixed,and enlarge the lattice ,the more number >> of plane waves will enter into the calculation ,and will get the lower >> energy. i do not understand what is the relationship between more plane >> waves and a lower energy . >> and just now you told me to set bigger encut and ..,seems opposite to the >> above discussion . >> maybe my understanding is wrong . >> >> i need your help . >> thanks in advance >> 2007/12/3, Hande Ustunel <hande at newton.physics.metu.edu.tr>: >>> > _______________________________________________ > Pw_forum mailing list > Pw_forum at pwscf.org > http://www.democritos.it/mailman/listinfo/pw_forum >
