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While performing molecular dynamics simulations on a co-polymer, how long should the minimum size of the polymer be, to replicate the experimental properties with a reasonable error?

Within this length, are there any standards to be followed for the order of the monomers (for example, -A-A-B-A-B- or any random arrangement), with a given composition of the total ratio A:B ?

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    $\begingroup$ That's going to depend a lot on the polymer and experiment. For example, experimentalists will sometimes know an expected "block length" (e.g. they might know it's unlikely to have more than 3-4 or .. 10 repeats of the same monomer). If you don't know that, and it's a random statistical co-polymer, you'll need to try a few compositions and block lengths and see if the properties change much. $\endgroup$ – Geoff Hutchison May 11 at 12:54
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This is a question that cannot have a "right" answer. As mentioned in the comments, the answer would depend on the experimental system and on the specific properties you want to reproduce.

Since most experimental systems are in what is known as the "scaling regime" you probably need long polymers to begin with (I commented on the issue elsewhere).

In addition to the length, when dealing with co-polymers the effect of the sequence can be dramatic, and hence you need to be extra-careful when choosing it. Here is a recent (open access) paper that shows the phase behaviour of short-ish (20 beads) co-polymers with different sequences. It also includes examples of polymers having the same AB composition but different arrangements. As you can see, different arrangements can give raise to sensibly different macroscopic behaviours.

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