Contribute PVA OPLS-AA/LigParGen .ff file

[ricalessandri]

This is a self-sufficient PVA ff file. Usage example to obtain a 10-mer:

polyply gen_params -f PVA.ff  -name PVA -seq CH3a:1 PVA:10 CH3b:1 -o PVA_n10_oplsLigParGen.itp

This is currently for this discussion.

This PR also brings up a "philosophical" question IMO. Should we try to have a "generic" -CH3 termini for OPLS polymers or should we have multiple custom "-CH3" termini for PS, PVA, PEO, etc. (in which case, how do we name them given that, if one wants to use the library, each block must have a different name)?

[fgrunewald]

In principle I think both input files are alright. However, one of the problems that I see is that there are two PVA blocks of which the atoms have actually a different charge. That's more problematic in my opinion than having the terminii specific to the monomers.

I also was finally able to complete the itp-to-ff code except for some small issues with charges, which in fact are quite asymmetric still. I wonder if we first should tackle the itp-to-ff and then update this PR? What do you think

[ricalessandri]

Very good point. Looking at the charges, the changes in the PVA block charges are not really significant. But indeed we should think about how to have a unique PVA block. Or, more generally, how to deal with this in general.

I agree that tackling itp_to_ff first and the fixing this PR is the way to go. I actually wanted to talk to you about itp_to_ff (also in relation with the latest GH discussion that involved this PR). I will write you an email so perhaps we can have a Zoom chat about it soon.