Difficulty in modeling water molecules(TIP3P) with polymer(COMPASS)

Hello friends,
I want to model the diffusion of water molecules in a polymer. I am using a class2 (COMPASS) forcefield for modeling the polymer and TIP3P for water molecules. I am using ‘hybrid’ pair, bond, and angle styles. I am getting the following error-

ERROR: Unexpected empty line in AngleCoeffs section (…/read_data.cpp:1847)

It seems LAMMPS requires cross-term parameters (BondBond and BondAngle) for water molecules. The angle type for water molecules has been defined in both data and input files as ‘harmonic’ but it seems it is considering water molecules as defined in class2 forcefield as well.

Kindly suggest


I doubt that you can use data file Coeff sections for your kind of system. Try to set those parameters with the corresponding coeff commands in the input.

Thanks for your suggestion.
I could not find cross-term parameters as part of TIP3P model but came across this page -

This page provides Bond Bond and Bond Angle Coeffs but as part of class2 forcefield.

Only for trial, I added these parameters to the data file to see its effect on the simulation. The error that I was facing was removed. But this doesn’t solve the problem as these are not part of TIP3P according to which other parameters are provided.

Here are the lines from the data file:

**Angle Coeffs # class2

1 class2 107.66 39.641 -12.922 -2.42999
2 class2 110.77 41.453 -10.604 5.129
3 class2 112.67 39.516 -7.443 -9.5583
4 harmonic 55.0 104.52

BondBond Coeffs

1 class2 5.3316 1.101 1.101
2 class2 3.3872 1.53 1.101
3 class2 0 1.53 1.53
#4 class2 2.4139579349904396e-23 1.0 1.0

BondAngle Coeffs

1 class2 18.103 18.103 1.101 1.101
2 class2 20.754 11.421 1.53 1.101
3 class2 8.016 8.016 1.53 1.53
#4 class2 0.228 0.228 1.0 1.0

Kindly suggest.


You do not need them. TIP3P does not use CLASS2 interactions. In fact, the bond and angle force constants are not relevant, since you must use fix shake on those bond/angle interactions as TIP3P is a rigid water model.

I already made a suggestion.

There is an error regarding missing angle coefficients when these are not provided in the data file. This is causing confusion.

Regarding the use of SHAKE-
I am trying to apply it but facing another difficulty despite providing the SHAKE command for the water molecule bond but the SHAKE stats (type/ave/delta/count) show that it is getting applied to a bond type in the polymer. I couldn’t locate the exact reason. I tried changing SHAKE fix to include bond and angle type, then I tried to include kbond value, also I tried the molecule file…but to no avail. But yes I have to use SHAKE once this class2 parameter problem is solved.


As I already mentioned, I don’t think that providing the Coeff sections in the data file will work for you. You should provide those values via the corresponding xxx_coeff commands in the input instead.

Without seeing your input, the corresponding output, and know which LAMMPS version you are using, it will be difficult to give any specific advice here.