How to define cations positon from CIF but hold its "cation attribute" in RASPA

[ykz]

Hi, I was read the RASPA doc and have some confuse in “Example 4: Adsorption of CO2 in Na-LTA” in Page 98 :

    1.the guide says "it is important to start from crystallographic position ", but the example use "RestartFile no". So I am confuse how to define position with my cif while keep it's "Cation attribute" in Raspa. Should I run a simulation to generate random cation position in a restart file, and then substitute the cations position from cif?
    2.the guide also says "use only translation for the ions" and "Reinsertion moves may transport the ions to positions in the windows and this is especially important for diffusion".  Does the "reinsertion" is one kind of "translation"（equal to randomtranslation here,right?)?  Since we already take a cation position from cif，why should we randomly change them positon to make the cif position meaningness?
    3.the Example 5 also says "the ions are read from the restart file" but the "RestartFile no" still exists below. Is that some mistake?

thanks for your time and I will appreciate it if someone response my question

[David Dubbeldam]

To reliably model adsorption/diffusion, the mobility of the cations needs to be taken into account. For example, in FAU there are many different cation-sites with certain "occupancy" ratios. Your cation model needs to be able to reproduce that.
That is why it is hard to reach an equilibrated system and it is a good idea to, when you have generated that, to store these positions in a restart-file.
Since cations are charged, the usual CBMC biasing can lead to numerical issues, but for single-atom cations you can achieve the same with a random-translation that tries to place the cation randomly in the box.
Look in the literature for papers on modeling of zeolites with cations for more pointers.

[ykz]

Thanks for your response.
So it is correct that I randomly translate the cation Na in FAU if my model can reproduce the certain "occupancy"

To reliably model adsorption/diffusion, the mobility of the cations needs to be taken into account. For example, in FAU there are many different cation-sites with certain "occupancy" ratios. Your cation model needs to be able to reproduce that.
That is why it is hard to reach an equilibrated system and it is a good idea to, when you have generated that, to store these positions in a restart-file.
Since cations are charged, the usual CBMC biasing can lead to numerical issues, but for single-atom cations you can achieve the same with a random-translation that tries to place the cation randomly in the box.
Look in the literature for papers on modeling of zeolites with cations for more pointers.

[David Dubbeldam]

You need a good force field to reproduce the correct occupancy. Also, the simulations are non-trivial and some people use parallel-tempering to compute the proper occupancy distributions.