How to have chain identification to have more than 70 chains in simulation box. This limits the options to 62 characters: A to Z, a to z, and 0 to 9?
How to solve it
If I remember correctly, pdb2gmx changes chain when the chain identifier changes between two subsequent atom entries. So actually 2 chain identifiers are sufficient.
pdb2gmx also separates chains at a TER record.
Got it. Thank you so much.