In data 08 giugno 2009 alle ore 14:47:08, ali kazempour <kazempoor2000 at yahoo.com> ha scritto:
> > Dear all > I want to study defect in TiO2. For a supercell with 2*2*3(72 atoms) > size , Do I relax the atomic position and lattice parameter > simultaneously (vc relax)or not?what about bigger supercell? Dear Ali, it depends on the system you want to simulate! In other words, if you assume the density of defects to be small, you'll expect the defective supercell to be forced, by the surrounding flawless crystal, to have the same lattice parameter as the unperturbed TiO2. E.g. if you remove a single Oxygen from the middle of a big TiO2 crystal it does not provide enough energy to force the entire crystal to change its lattice parameter, there may be a local strain which you account by taking a big enough supercell. As periodic-boundary condition always imply a relatively high density of defects (one every 12 unit cells, in your case) you have to use the lattice parameter of unpertubed TiO2, and only relax the atoms positions (which will move to compensate the local strain). The system will actually feel a stress, but that's exactly what's happening in the experiment. On the other hand, if you want to study a system with a high density of defects, in your case about 1 every 12 unit cells, than you have to relax the cell parameters together with the ionic positions. regards -- Lorenzo Paulatto SISSA & DEMOCRITOS (Trieste) phone: +39 040 3787 511 skype: paulatz www: http://people.sissa.it/~paulatto/ *** save italian brains *** http://saveitalianbrains.wordpress.com/
