Dear LAMMPS community,
I’m using fix gcmc to insert flexible SPC water molecules (constrained by fix shake) into 3d periodic boxes at 300K to simulate sorption in a 4 nm-width silicate slit pores with 3 nm walls at saturation, with the 29 Sep 2021 version of LAMMPS. The “full_energy” option is enabled since the pair style is lj/cut/coul/long and PPPM solver used. The chemical potential I used was acquired by adjusting the molecule file and the value of mu in the example “in.gcmc.h2o” script and see when the liquid phase formed (about -10.5 kcal/mol, which is near to literature value). The insertion of water molecules in pore wall structures and on walls surfaces are working, but I expect full filling of the pore space, rather than a few layers adsorbed to the surface. I tried adopting -8 kcal/mol for chemical potential, and the full filling occurred, but this value seemed to be way too high. I have also tried original rigid SPC model, but it doesn’t work better. A large number of MC steps adopted because the initial configuration should be filled with water.
My questions are:
(1) Whether I had got the value of chemical potential wrong? Should I use a different chemical potential or adopt another way for deriving chemical potential? The exact saturation vapor state is important to me because I want to simulate sorption.
(2) I found that a previous thread in the mailing list stated that with “full_energy” option on, “the large and positive intramolecular energy which causes insertion events to have a very low probability of acceptance”, and the “intra_energy” option is thus provided. I am not sure that whether this case is suitable for SPC water models (rigid/flexible with fix shake). If so, how should I calculate and specify this “intra_energy” value?
(3) Another confusing issue is that some slightly different silicate pore wall (3 nm, slightly different ion distribution, file A and B) with unchanged geometry (4 nm void) may induce pore filling. In most cases the same scenario occurs (like in file A), but in a few cases (file B, rare, no regularity found), a lot of water molecules will quickly appears in the pore void (even with mu of -10.9 kcal/mol). Why is this happening? The further nvt relaxation or minimization of pore wall do not relieve the problem.
(4) Is there anything else I can do to solve this problem?
I attached two .lammpstrj and .log file to this e-mail. The file A and file B are same in wall composition, but little different in distribution of ions due to random selection. File A has a mu of -10.5 kcal/mol, but no full filling observed; while file B has a mu of -10.8891 kcal/mol, but full filling can be observed on the very first step.
Much thanks in advance!
Zhen
fileA.lammpstrj (782 KB)
fileB.lammpstrj (950 KB)
fileA.log (7.07 KB)
fileB.log (11.2 KB)