[CP2K:4585] IR of H2O molecule

[Ari Paavo Seitsonen]

Dear Jianjun,

  Does your molecule rotate? This is usually the reason for peak splitting
that I observe. One could prevent both the translation and rotation of the
molecule with the option in the section &MOTION ... &MD ... &END MD ...
&END MOTION ("ANGVEL_ZERO" and "COMVEL_TOL" I think).

  Please notice that your choice of the cut-off energy, 280 Ry, is most
likely insufficient (this was the historical value). I don't have much
experience on the MOLOPT basis for oxygen, but you might need some 350-400
Ry (with the TZV*P it is even more). Also if you make "EPS_SCF" tighter you
might want to do the same for "EPS_DEFAULT".

    Greetings from Zurich,

       apsi


2013/8/27 JianJun Yang <jjyan... at gmail.com>

> Hi All,
>
> I am trying to learn how to calculate the IR spectrum with cp2k, and now I
> am testing on the most simple sample --- a single H2O molecule. The MD run
> was done within the NVE and the simulation time was over 30ps with a
> timestep of 0.5fs. The IR is obtained through the calculated wannier
> centers by applying the TRAVIS program. As we know, a single water molecule
> only has 3 vibrational modes: 2 stretching modes and 1 bending
> mode. However, the IR spectrum I obtained  does not agree with that well.
> See below for details.
>
> (i) The IR shows the bending mode split into two, and the stretching modes
> split also. I tried to increase the SCF convergency criterion, or tried the
> NVT ensembles,  but none of them worked. There must be sth wrong, but I
> cannot figure it out. Could anybody help me out? Thanks a lot!
>
> (ii) At the same time, in my simulation box (length of 10 A), along the MD
> time, the molecule is always drifting. When I increase the EPS_SCF from the
> default value to 1E-7, it got better but still drift dramatically. Any
> comment on this?
>
> For your reference, here attached the input file and the corresponding IR
> plot. And the cp2k I used is the version 2.4.0.
>
> Thanks again,
>
> Jianjun
>
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-- 
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  Ari P Seitsonen / Ari.P.S... at iki.fi / http://www.iki.fi/~apsi/
  Physikalisch-Chemisches Institut der Universität Zürich
  Tel: +41 44 63 55 44 97  /  Mobile: +41 79 71 90 935
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