How to use SETTLE constraint for tip4p water model?

Dear all

I want to use SETTLE constraint for my tip4p water model for solvation of a kind of PPO polyelectrolyte and study water structure and diffusion coefficient. I saw tip4p.itp in OPLS folder and there were defined as:

#ifndef FLEXIBLE
[ settles ]
; OW funct doh dhh
1 1 0.09572 0.15139
and …

Then I just simply added
define = -DFLEXIBLE

in my mdp file for min.mdp, nvt.mdp and npt.mdp.

I want to know first, is it a true way to define SETTLE constraint on water? and second, do I have to use for all min, nvt and npt level? Somewere was recommended to use for minimization first with -DFELEXIBLE and second without it. its abit confusing for me.

Thank you

Rigid water via SETTLE is the default. If you use define = -DFLEXIBLE you are explicitly disabling SETTLE and using a flexible water model. Note that #ifndef is the opposite of #ifdef.

Thank you very much for your response.