# Constraining two water droplets at constant distance between their COMs using pull code

**URL:** <https://gromacs.bioexcel.eu/t/constraining-two-water-droplets-at-constant-distance-between-their-coms-using-pull-code/6329>\
**Category:** User discussions\
**Tags:** mdp-parameters, umbrella-sampling, pull-code\
**Created:** [May 3, 2023, 9:53am UTC](https://gromacs.bioexcel.eu/t/constraining-two-water-droplets-at-constant-distance-between-their-coms-using-pull-code/6329 "2023-05-03T09:53:02Z")\
**Posts on this page:** 1\
**Showing post:** 22

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**Author:** ![tonmoy](https://avatars.discourse-cdn.com/v4/letter/t/e36b37/32.png) [@tonmoy](https://gromacs.bioexcel.eu/u/tonmoy)\
**Post date:** [May 6, 2023, 9:58am UTC](https://gromacs.bioexcel.eu/t/constraining-two-water-droplets-at-constant-distance-between-their-coms-using-pull-code/6329/22 "2023-05-06T09:58:17Z")

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In the previous results, the temperatures extracted from the velocities of the atom groups needed to be corrected based on the no. of constraints imposed.

The **command used** for extracting the temperature values from the trajectory velocities: gmx\_mpi traj -f nvt.trr -s nvt.tpr -ot temp\_SOL1.xvg -w -b 0 -e 5000

**Correction needed:** T\_real = T\_calculated \* [3_natoms / (3_natoms - nconstraints)] (Reference: [How can I obtain the temperature of a specific group?](https://gromacs.bioexcel.eu/t/how-can-i-obtain-the-temperature-of-a-specific-group/3423))

Here

natoms = no. of atoms in the water molecule = 4  
nconstraints = no. of constraints = 6 (3 for SETTLES in the topology file of water molecule + 3 for the constraining all the bonds by LINCS in MDP file)  
T\_real = T\_calculated x 2

The corrected temperature profile is shown in the image below which matches with the set temperatures.

 ![Combined analysis](https://europe1.discourse-cdn.com/flex017/uploads/bioexcel1/original/2X/6/64e1b295d3dcd0f642d895862881b50984142017.png)

For the second simulation, as the temperature of both droplets are maintained at 300K. there is no evaporation and no fluctuations. However, the droplet splits into 2 over a longer time.

Kindly suggest as to what might be the cause of the droplet splitting.

Thanks and regards,  
Tonmoy Sharma

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