methane in a water box QM/MM

Axel akoh... at
Wed Mar 18 15:43:30 UTC 2009

On Mar 18, 11:12 am, Marius Retegan <marius.s... at>
> 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 ( 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
hydrogens rigid, an MD with a time step larger than 0.5fs will not run
in any code without doing unphysical tricks, or switching to langevin
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
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 ;)).


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

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