[Pw_forum] Geometry Optimization

[Michael Sullivan]

Prof Paulatto:

Very good point.

Mahasin:

Listen to the master. :)

For slabs, I usually optimize the bulk cell and use that lattice constant to generate a surface. With this surface, I don't change the cell (ie I use calculation='relax').

Hope that helps and sorry for the confusion.

Mike
michael at ihpc.a-star.edu.sg
http://www.sullivan.sg/


-----Original Message-----
From: pw_forum-bounces at pwscf.org [mailto:pw_forum-bounces at pwscf.org] On Behalf Of Lorenzo Paulatto
Sent: Thursday, August 06, 2009 5:03 PM
To: PWSCF Forum
Subject: Re: [Pw_forum] Geometry Optimization

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/

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