[Pw_forum] questions about vc-relax or geomtry

[Eduardo Ariel Menendez P]

Hi,
I take the occasion to ask some clarifying on the variable-cell
optimization. Maybe it seems difficult because is not well documented.
After some laborious vc-relax calculations with small unit
cells, the optimal values of wmass are still misterious for me.

When I follows the the instructions in INPUT_PW

wmass          REAL ( default = sum of atomic masses in the cell )
               fictitious cell mass [amu] for variable-cell simulations
               (both 'vc-md' and 'vc-relax')

then I see that the cell varies extremely slowly. Then I have chosen
"sum of atomic masses in the cell"/Factor using Factor as 100,1000,etc
until I see and effective change in volume. When it happens I see the volume
converges fast, but then the shape began to fluctuate for a long time
(maybe this is the moment to use Factor=1).
If wmass is not specified pw provides a value that
is reported in the output and is much smaller than "sum of atomic masses
in the cell".

For example, a Pt in HCP structure, with two atoms per cell as
"sum of atomic masses in the cell"=2x195.084=390.168

the default reported by pw when wmass is unset is "0.00300 AMU/(a.u.)^2"
There is a factor of 130000, which I do not understand. ¿What is a.u. in
AMU/(a.u.)^2 ?

The units of W seems to be E*T^2/V^2 (E=Energy,V=Volume,T=time) in the
Andersen and Parrinello-Ramman Lagrangians.
http://www.vlab.msi.umn.edu/events/download/vlab_lectures/renata/lecture1.pdf
Is the factor related to the volume?
In my case of Platinum el volumen es 203.4(a.u.)^3., so V^2=41371 which is
near 130000/3.

Hence, some relation is betwwen the mass in AMU, the volume, and maybe the
time step. ?Can someone explain it?

Second, it seems that one value is appropriate for volume relaxation, and
other for shape relaxation, depending on the elastic constants. I
welcome  know-how sharing.

Third. In damped dynamics, there should be some damping factor, but it
seems to be fixed in PWSCF. Is it?

Eduardo A. Menendez Proupin
Department of Physics
Faculty of Science
University of Chile
Las Palmeras 3425
Ñuñoa, Santiago
Chile
Phone: (56)(2) 978 74 11
Fax  : (56)(2) 271 29 73
http://fisica.ciencias.uchile.cl/~emenendez/




? 2007-08-19?? 20:44 -0400?Axel Kohlmeyer???
> On Mon, 20 Aug 2007, jideson wrote:
>
> J> Dear experts,
>
> dear jideson,
>
> J>      I read some  emails about vc-relax in this list, but  still have
> J> some questions.  It seems that  geometry optimization (both cell and
> J> atomic postions) in pwscf is a diffcult job than other softwares such
as
>
> i disagree. it is not difficult, you just need to know what you are
> doing and particularly know what you can expect. the quantum espresso
> package allows you a lot of flexibility, but the price for that is
> that not everything works automatically right. usually, if something
> is not working easily, it is an indication, that you have not yet
> understood what you are doing (or else it would be obvious) or
> you have a problem that is not easy to start with or (which is rare)
> have found a bug in the code.
>
> J> CASTEP.  Can you give me a step-by-step tutorial ?
>
> there are some input examples within the distribution
> and you may want to check out material from previous QE
> tutorials, e.g., the material at:
> http://www.vlab.msi.umn.edu/events/lecture.shtml
>
> J>    By the way , another question , are there differences between cp.x
> J> and cpmd(www.cpmd.org) code ?  Thanks!
>
> yes. both have the same origin, there have evolved into
> quite different species by now.
>
> cheers,
>    axel.