[gmx-users] how to arbitrarily increase the XY size of an existing bilayer system (adding more lipids)

[Christopher Neale]

Dear Users:

I have some atomistic systems of a membrane protein embedded in a lipid bilayer. I currently have N lipids and I would like to increase that to 1.5N or 2N lipids (distributed equally in the bilayer plane) without disturbing the existing structure. Increasing to 4N lipids would be relatively easy by tiling the exisintg lipids 2x2 in XY. I am writing to ask if anybody has ever done this and if they have any suggestions.

The ideas that I have at present are:
a) add lipids and equilibrate (using restaints on existing lipids and protein). Increase in size could be by simply increasing XY dimensions and then using gmx genbox, knowing that I will end up with a bilayer defect at the edges that I will have to heal by MD.
b) convert to a CG model, add lipids and equilibrate, then back to an AA model (using restaints on existing lipids and protein)

I realize it sounds like an absurd requirement, but for scientific reasons in this particular case I can not simply start over by embedding the protein in a new, larger bilayer (i.e., it’s not just about saving compute time when generating the larger systems).

Thank you for any advice,
Chris.