[gmx-users] Which method for choosing concentration is more appropriate

[Apramita Chand]

Dear All,
This is a general query regarding solvation of peptide in urea-water.
My method is to place the peptide in the box first, add urea molecules
using g_genbox and then solvate it using water molecules to maintain a
desired concentration.
There can be two approaches to this:
1) I can keep no.of water molecules constant and increase the urea
molecules like 15,30,45 and so on.
Here total number of particles will not remain constant

2) I can keep total number of particles constant. Say I have 999 water
molecules initially and then I add 15 molecules of urea and the system
becomes like 985water-15urea and then subsequently 969water-30urea.
But wouln't the effect on peptide hydration will be biased in terms of
which is actually causing decrease/increase in hydrogen bonding with water:
decrease of water or addition of cosolute?

In many papers, I have seen that the no.of water molecules decreases with
addition of cosolute.
Is there any drawback if  I use the first method?

Yours sincerely
Apramita