[gmx-developers] tpic with large cavities

[hess at sbc.su.se]

> Hi all,
>
> Currently I'm looking into the cavity test particle insertion (TPIC) code
> since I want to make a modified version of the algorithm. Due to obtaining
> strange results I started to do some testing of the original tpic
> algorithm (using the current git version). My test system is a plain water
> box into which I insert a helium atom. I calculate the energy in a
> spherical cavity at the center of the box and when the radius of insertion
> (rtpi in the mdp file) is increased I expect the energies to converge
> towards the energy calculated using the standard TPI algorithm for the
> whole box.
>
> Small radii gives quite accurate energies, but large radii give much lower
> energies than expected. A clue might be that the threshold for my system
> lies around the size of the cutoff. When I tried to debug the code I
> noticed that (as far as I can see) do_forces are using the center of the
> cavity instead of the actual insertion point. But setting
> "fr->cg_cm[top->cgs.nr-1]" to "x_tp" instead of "x_init" did not help.
>
> This should not be a sampling problem since insufficient sampling would
> overestimate the energy rather than underestimate it. (And the energies
> seems to converge nicely too.)
>

I guess your problem problem is that r_tpi is large compared to rlist.
As the manual says, neighbor searching is done only once per step.
The neighborlist is made with respect to the center of the cavity,
but the energies are determined for the actual location of the molecule.

> Another problem in the TPIC code is that the box volume is used to
> calculate the energies instead of the volume of the insertion sphere
> (which is constant and cancels out in the equation). This might give large
> errors if the box volume changes a lot due to pressure rescaling.

The volume issue is always tricky.
This corrects for the probability of encountering the certain part
of space where you are inserting. We should not correct for the volume
of the whole box. But I don't see immediately if and if so,
what type of volume correction is required for tpic.

Berk

>
> Ideas and comments are most welcome!
>
> Best regards,
> Daniel
>
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