IR of H2O molecule

[JianJun Yang]

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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