methane in a water box QM/MM

[Axel]

On Mar 18, 11:12 am, Marius Retegan <marius.s... at gmail.com>
wrote:
> Hi,
>
> If I change QMMM with FIST the input runs.
>
> I also have some additional questions, but on somewhat different matters.
>
> In the latest papers that I could find that use cp2k to do qm/mm
> calculations (http://dx.doi.org/10.1021/ja076081handhttp://dx.doi.org/10.1021/jp074858n) the authors used a integration step of
> 0.5 and 0.48. Unfortunately, they did not discussed their choice. I'm used
> to running qm/mm md with other packages, but I've always used an integration
> step of 1 fs. Is cp2k different :-)?

unless you make _all_ bonds (including the ones in the QM region) that
contain
hydrogens rigid, an MD with a time step larger than 0.5fs will not run
stable
in any code without doing unphysical tricks, or switching to langevin
dynamics
with a significant friction coefficient.
for non-deuterated, unconstrained hydrogens already 0.5fs is pretty
much at
the limit in my experience and if, for example, you have a proton
transferred
across a large barrier, you may get instabilities because of the
proton accelerating
too much. to also give a lower barrier, i found 0.25fs to be a very
safe and
conservative choice (in more than one way ;)).

cheers,
    axel.

>
> Is there any place where I can find information regarding the parameters
> xc_deriv and xc_smooth_rho and are "good" choices?
>
> Thanks,
> Marius