GCMC

Isaak Daniels isaakdan... at gmail.com
Fri Aug 26 20:07:39 UTC 2011

```And one would set it by using the Pressure?

Thank you

On Aug 26, 11:12 am, Isaak Daniels <isaakdan... at gmail.com>
wrote:
> Is there a way to find the average number of molecules in each box for
> a GEMC simulation?
>
> Thank you
>
> On Aug 25, 2:23 am, Matt McGrath <obfis... at gmail.com> wrote:
>
>
>
> > The only way to set thechemical potentialis if you use the above
> > tricks and they work.  You can't set it directly, but you could choose
> > your ideal gas to have a certainchemical potential.  GEMC would
> > equilibrate thechemical potentialbetween the two boxes, and provided
> > your ideal vapor box is big enough (i.e. a true reservoir), its
> >chemical potentialwould not change, so therefore thechemical> potentialin both boxes would be (approximately) the same as what you
> > started with for your ideal gas.  Finding what is "big enough" would
> > probably take some effort, though, and there would be error bars
> > associated with the number, so this is not a perfect solution.
>
> >                                    Cheers, Matt
>
> > On Aug 24, 9:21 pm, Isaak Daniels <isaakdan... at gmail.com> wrote:
>
> > > Is there a way to set achemical potential, such as -454?
>
> > > Thank you
>
> > > On Aug 23, 10:08 pm, Matt McGrath <obfis... at gmail.com> wrote:
>
> > > > When you say "import coordinates of a crystal", do you mean as a swap
> > > > move?  As in, you want to have gas molecules and crystal molecules
> > > > come into the box, or that you put crystal coordinates into the box at
> > > > the beginning, treat the whole box as QM, and then swap gas molecules
> > > > into the box (also treated as QM)?  The latter option should be
> > > > possible if the test I described above worked (since the force_env of
> > > > each box can be specified separately), and is what I assumed you
> > > > wanted to do in the first post.  If you also want to swap in crystal
> > > > nuclei, you could do that by making it a new molecule type (and
> > > > creating a psf file for it), but then it would swap into the QM box in
> > > > a completely random position, and would still be treated as a molecule
> > > > from CP2K's standpoint (rotated and translated together).  I don't
> > > > know if that's what you want or not.
>
> > > >                       Cheers, Matt
>
> > > > On Aug 23, 9:23 pm, Isaak Daniels <isaakdan... at gmail.com> wrote:
>
> > > > > Can I also have it so that in the non-vapor box, gas molecules can
> > > > > enter that box and one can also import coordinates of a crystal, with
> > > > > this box having QM?
>
> > > > > Thank you
>
> > > > > On Aug 23, 3:21 am, Matt McGrath <obfis... at gmail.com> wrote:
>
> > > > > > Hmm.  I was thinking about that, and I didn't think it would work
> > > > > > before I responded to your original question.  But now, I'm not so
> > > > > > sure.  If you set all the MM interactions in the vapor box to have
> > > > > > epsilon=0 and no charges (so you don't need Ewald sums), then you have
> > > > > > an ideal gas, so...maybe.
>
> > > > > > A quick way to check would be to try it with MM in both boxes, but set
> > > > > > all the LJ parameters and charges equal to zero in the vapor box.
> > > > > > Choose it to be large enough that the density doesn't really change
> > > > > > over the course of the simulation, don't do any molecule displacements
> > > > > > in the vapor box, and compare it against GCMC with a different code.
> > > > > > One concern I have is that, on swap moves, the code is going to check
> > > > > > for overlaps in the vapor box, which it should do for true GCMC.  If
> > > > > > you try this test and get an answer that is close, and you feel
> > > > > > comfortable changing the source code, I can let you know which lines
> > > > > > to change to remove those checks so we can see if it works.  I know of
> > > > > > codes that do GCMC as basically GEMC with a box that has no
> > > > > > interactions in it (and is big...500 molecules or more), so this
> > > > > > doesn't seem so different.
>
> > > > > >                                               Cheers, Matt
>
> > > > > > On Aug 22, 10:57 pm, Isaak Daniels <isaakdan... at gmail.com>
> > > > > > wrote:
>
> > > > > > > Can one do it so that the box not of interest (representing the
> > > > > > > environment) is MM while the other is done with QM, so one can avoid
> > > > > > > the pitfalls of enlarging the box?
>
> > > > > > > Thank you
>
> > > > > > > On Aug 21, 7:42 pm, Matt McGrath <obfis... at gmail.com> wrote:
>
> > > > > > > > Hi Isaak.  Unfortunately, this isn't really possible at the moment,
> > > > > > > > unless you do Gibbs ensemble Monte Carlo and make the second box huge,
> > > > > > > > i.e. essentially a reservoir.  Of course, if you're using any grid-
> > > > > > > > based method to compute the energy (Quickstep, Ewald) this adds a
> > > > > > > > whole lot of expense to the simulation as well, and probably isn't
> > > > > > > > worth it.
>
> > > > > > > > GCMC has been in the pipeline for a while (there have been issues with
> > > > > > > > the choice of reference state for QS calculations with GCMC), and is
> > > > > > > > the next big project on my list.  I'm about 30% done with the current
> > > > > > > > project (making the MC routines more streamlined), so it might get
> > > > > > > > implemented in the next year...but that's just a guess.  Could be six
> > > > > > > > months, could be a couple years, so if you're working on a major
> > > > > > > > project, it's probably best to use a different code.  Sorry!
>
> > > > > > > >                                 Cheers, Matt
>
> > > > > > > > On Aug 21, 2:51 am, Isaak Daniels <isaakdan... at gmail.com> wrote:
>
> > > > > > > > > Is there a way to run a sort of GCMC on Cp2k?
>
> > > > > > > > > Thank you
```