Is this the correct way of using two potentials in the same input script.
#for Al,Si, Mg
pair_style meam
pair_coeff * * library.meam AlS SiS MgS CuS FeS AlSiMgCuFe.meam AlS SiS MgS NULL
#For Fe
pair_style eam/fs
pair_coeff * * Fe_2.eam.fs Fe Fe Fe Fe
Try it and see 
More importantly, think about the physics of what you’re doing. Otherwise you aren’t running a useful simulation – you’re just cranking out antirandom numbers.
The initial phase of the simulation is running without any errors. However, before diving into the more complex part, I want to ensure that the applied potentials are being implemented correctly. Based on previous discussions in the forum, applying two potentials simultaneously in a single input does not seems to be a great idea as it can lead to bogus results.
I would appreciate any suggestions on how to verify the correct application of potentials.
That only means that there are no syntax errors. LAMMPS cannot tell whether your model is meaningful or bogus. It just computes and applies the forces as you tell it to do.
Your input is syntactically correct, but does not do what you seem to be attempting to do.
Your input will simulate a system where all atoms use the iron EAM potential.
The second pair_style command will wipe out the previous one. You can have only one active pair_style setting. For using multiple different pair styles in the same simulation you would need to use pair style hybrid, but that is not as simple as it may sound. You cannot do something where you would say “simulate all atoms with this one potential and only one element with a different potential”. For this you also need to provide a model and parameters for the cross interactions. This is effectively not possible with any kind of manybody potential, so you would need to use a pairwise additive potential for that like Lennard-Jone or Morse (which are often not a good fit). For many-body potentials this “mechanical coupling” will also suppress the manybody interaction terms between the different partitions. For EAM and MEAM that means the embedding term. The two subsets simply do not “see” the contributions from the other atoms to the embedding energy and thus the forces are missing this.
This leads to the logical conclusion, that using a hybrid pair style for manybody potentials is rarely a good idea and the error is the smallest when you have two different subsets, e.g. when nanomachining a metal slab (described by EAM) with a diamond tool (described by Tersoff) and then you mostly have repulsive interactions as cross-term which can be represented by Morse or ZBL.
To test your setup, you need to run simulations that would reproduce known properties of the material in question.
It is not, and particularly not for a case where you already have a potential that does cover all the elements you need. Why do you not want to use that one?
[quote=“akohlmey, post:4, topic:54120”]
Why do you not want to use that one?
[/quote] I couldn’t find a single potential file that covers all the elements Al, Si, Mg and Fe. The available potential file, AlSiMgCuFe.meam, includes all these elements, but there’s no evidence of using in Fe-Fe interactions previously. Therefore, I had to use a different potential specifically for the Fe-Fe interaction.
That is much worse an approach than just using the MEAM potential for the reasons I mentioned.
Why not make some tests with the MEAM potential for pure iron to see how well it represents iron.
When you are trying to mix and match different potentials, you always have to treat it as if it was a new potential altogether and run tests.
Please also note that these kinds of discussions are only marginally relevant to this forum here. That is something you need to discuss with your adviser/tutor. Nobody here has the time to always fill in for these kinds of discussions and become your surrogate adviser. It’ll happen occasionally at the beginning, but you’ll quickly see interest in helping you wilting away. … and that is a big risk in case you would need some help with a problem that is on-topic for this forum.
Please also note that questions of the “please tell me if this is correct” fall into a similar category. You would need to acquire the skill to construct tests to check this for yourself. That is how research works and (again) that is the job of your adviser/tutor to teach you, not that of an online forum.