Unphysical pressure increase during FFS (cavitation)

[ProtKsen]

Dear developers of SSAGES, we kindly ask you to answer on an issue, appearing during using SSAGES.

After successful installing, built with LAMMPS, and repeating the example we tried to use FFS method to investigate a cavitation in NPT ensemble. We choose a volume of the whole system as a reaction coordinate (cv). SSAGES properly calculated the first flux and generated collection of starting configurations.

However, when SSAGES initiated the trial runs, the system pressure dramatically increases (step 1019908), this leads to the big change of thermodynamical conditions (lost metastability).

Please, tell us, is it a normal situation? Or it happens because of the badly chosen reaction coordinate?

1019897 0.50316223 -0.85509203 -5.7866565 -5.0319886 12123.362 1019898 0.50343554 -0.85733503 -5.7870077 -5.0319299 12123.557 1019899 0.50368328 -0.8594207 -5.7873267 -5.0318774 12123.748 1019900 0.50390648 -0.86154426 -5.7877333 -5.0319491 12123.934 1019901 0.5041061 -0.8634115 -5.7880485 -5.031965 12124.114 1019902 0.50428271 -0.86501447 -5.7882676 -5.0319192 12124.289 1019903 0.50443697 -0.86651781 -5.7884905 -5.0319107 12124.458 1019904 0.50456939 -0.86777373 -5.788627 -5.0318486 12124.621 1019905 0.50468054 -0.86902895 -5.7888266 -5.0318815 12124.778 1019906 0.50477144 -0.86995783 -5.7888899 -5.0318085 12124.93 Iteration: 19906, proc 0 Successful attempt. (cvvalue_previous: 12124.9 cvvalue 12125.1 )

of successes: 10

required # of configurations: 10 Initial flux calculation was successfully completed FFSConfigIDQueue has 8 entries upon initialization 0 0 1 0 0 1 0 1 0 7 0 0 7 0 2 0 4 0 0 4 0 3 0 5 0 0 5 0 4 0 2 0 0 2 0 5 0 0 1 0 0 0 6 0 6 0 0 6 0 1019907 0.50122858 -0.87380454 -5.7889757 -5.037208 12125.075 1019908 0.50104887 0.1554287 -2.1753112 -1.4238131 12125.214 1019909 0.50083568 0.15682564 -2.1750072 -1.4238288 12125.437 1019910 0.50066708 0.15769521 -2.1747618 -1.4238362 12125.744 1019911 0.50054645 0.15804805 -2.1745622 -1.4238176 12126.136 1019912 0.50047654 0.15779738 -2.1744566 -1.4238169 12126.613 Iteration: 19912, proc 0 Successful attempt from interface 0 l0-n0-a0 (cvvalue_previous: 12126.6 cvvalue 12127.2 interface[1] = 12127

[hsidky]

Hi,

Based on what I can see above I believe there may be a couple of issues:

1) System volume may not be a good CV for cavitation which you are studying. You may need something that can reliably distinguish between whatever (meta)stable states you are interested in with minimum degeneracy of uninteresting states.

2) Free energy barriers in volume can be pretty massive. This may cause trajectories to double back after crossing the first interface. This is not unexpected if you have a very steep free energy gradient.

I don’t know the details of your system so I cannot speak precisely to what you are trying to study. However, I would suggest that you try to define the interfaces much closer together, and choose reasonable end points. This may solve the technical problem with FFS failing, but still may not solve your issues if the reaction coordinate is not ideal.

[ProtKsen]

Hello! Thanks for your reply. But there is another problem.. We are considering a cavitation in a system of particles interacting via Lennard-Jones potential and use the volume of the whole system as the CV. The number of walkers is equal 1, FFS method is used. Now we get an error when running the SSAGES program on several processors, say 8, after successful initial flux calculation in parallel mode. Below the input and output files are attached. Could you please shed a light what is the possible reason for this and the ways of the solution?

cav_npt.txt slurm-1426615.txt