In data 06 agosto 2009 alle ore 10:54:12, MAHASIN ALAM <mahasinalam at yahoo.co.uk> ha scritto: > But if I have a system with vacuum cell and if I need to relax the ions > as well as vacuum cell, can I do it using PWSCF? If yes can any one tell > me what I have to set in calculation type? Thank you for your help.
You can use vc-relax to relax both the ions and the unit cell (mainatining its symmetry); but it does NOT do what you want. What you want to do is find the smaller amount of vacuum that guarantees a negligible interaction between periodic copies along the vacuum direction. This is equivalent to minimize forces between peridic images, but does not keep stress in account and will likely /increase/ the total energy. On the other hand, vc-relax only tries to minimize stress and minimize energy. In practice, you have to do a few tryes with different amounts of vacuum and see when the total energy becomes stable. This may reuire as little as 5 Angstrom of empty space, for a simple slab geometry, but may be much more if you have dipolar molecules. best 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/
