How to Confirm if SETTLE Algorithm is Applied in TIP4P Water Model Simulation?

User discussions
1

GROMACS version: gmx2022.4
GROMACS modification: No

I’m trying to confirm if the settles algorithm is correctly applied in my TIP4P water model simulation. I’m running a simulation with 512 TIP4P water molecules using the following steps: EM → NPT → NVT.

I noticed that the results look nearly identical when I define [constraints] in the .itp file versus when I define [settles] in the .itp file.

I’ve attached my .itp and .mdp files for reference.
[ defaults ]
; nbfunc comb-rule gen-pairs fudgeLJ fudgeQQ
1 3 yes 0.5 0.5

[ atomtypes ]
; Include forcefield parameters
IW IW 0 -1.1128 D 0.0 0.0
OWT4 OWT4 15.9994 0.000 A 0.31589 0.77490
HWT4 HWT4 1.0079 0.5564 A 0.00000E+00 0.00000E+00

[moleculetype]
; name nrexcl
SOL 2

[atoms]
; nr type resnr residu atom cgnr charge
1 OWT4 1 SOL OW1 1 0 15.9994
2 HWT4 1 SOL HW2 1 0.5564 1.0079
3 HWT4 1 SOL HW3 1 0.5564 1.0079
4 IW 1 SOL MW4 1 -1.1128 0.0

[ settles ]
; OW funct doh dhh
1 1 0.09572 0.15139

[exclusions]
1 2 3 4
2 1 3 4
3 1 2 4
4 1 2 3

; e.g.: -DPOSRES -DFLEXIBLE (note these variable names are case sensitive)
; OPTIONS FOR BONDS
constraints = h-bonds
; Type of constraint algorithm
constraint_algorithm = lincs
; Do not constrain the start configuration
continuation = no

2

If you use [ settles ] SETTLE will be used.

Note that your constraint setup is incorrect. TIP4P is full rigid, so you should use constraints = h-angles or all-angles.

3

Thank you so much I truly appreciate your help