Epoxy Stress-strain

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1

Hi there,

We are attempting to obtain stress-strain data for a cross-linked epoxy. We have the cross-linked structure as a .pdb file which we convert to a LAMMPS .data file (atom style “charge”) and use the Liu et al. ReaxFF force field.

Our main question is that when converting from a .pdb file to a LAMMPS .data file (for ReaxFF), should we include the bond information (atom style “full”)? We believe no we should not include bonds since ReaxFF is bond-ordered based and all this information would be calculated in the force field. As a test, we ran the script using “full” style in the data file and the script, but we obtained an error with checkqeq convergence which leads us to believe “charge” would be the correct style. However, when using “charge” style the stress-strain results are less than optimal and have no correlation (think random scatter) using a similar script that provided great results for metallic systems.

Are we doing something wrong when converting from the .pdb to LAMMPS data file that is leading to this random scatter stress strain curve, or is it that we should also have a bonds section in our ReaxFF file since the epoxy is crosslinked?

Thanks for your help, the pairstyle and a sample of the data file are included below.

Stephen

pair_style reax/c NULL lgvdw yes

pair_coeff * * ffield.reax.lg C H N O

Sample of Data file:

Starting on line 3

468 atoms

0 bonds

0 angles

0 dihedrals

0 impropers

4 atom types

0 bond types

0 angle types

0 dihedral types

0 improper types

-0.318 15.240000 xlo xhi

0.047 14.957000 ylo yhi

-0.075 15.231000 zlo zhi

Pair Coeffs

1 C

2 H

3 N

4 O

Masses

1 12.010700 # C

2 1.007940 # H

3 14.006700 # N

4 15.999400 # O

Atoms # charge

1 1 0.000000 5.289000 12.519000 12.957000 # C

2 1 0.000000 5.259000 12.068000 14.301000 # C

3 1 0.000000 5.125000 10.680000 14.463000 # C

4 1 0.000000 5.054000 9.820000 13.345000 # C

5 1 0.000000 5.138000 10.349000 12.017000 # C

6 1 0.000000 5.360000 11.724000 11.765000 # C

7 3 0.000000 5.416000 12.991000 0.399000 # N

8 1 0.000000 5.076000 10.088000 0.847000 # C

9 2 0.000000 5.027000 8.734000 13.458000 # H

2

ReaxFF doesn’t require bond information.

Stress-strain response of epoxy would depends on – ReaxFF parameters, degree of cure, temperature, strain rate etc.

Sanjib

3

Hi there,

We are attempting to obtain stress-strain data for a cross-linked epoxy. We have the cross-linked structure as a .pdb file which we convert to a LAMMPS .data file (atom style “charge”) and use the Liu et al. ReaxFF force field.

Our main question is that when converting from a .pdb file to a LAMMPS .data file (for ReaxFF), should we include the bond information (atom style “full”)? We believe no we should not include bonds since ReaxFF is bond-ordered based and all this information would be calculated in the force field. As a test, we ran the script using “full” style in the data file and the script, but we obtained an error with checkqeq convergence which leads us to believe “charge” would be the correct style. However, when using “charge” style the stress-strain results are less than optimal and have no correlation (think random scatter) using a similar script that provided great results for metallic systems.

using atom style charge is the correct choice. having explicit bonds in the data file with ReaxFF is a mistake except for some very unusual corner cases, which require special settings, too.

Are we doing something wrong when converting from the .pdb to LAMMPS data file that is leading to this random scatter stress strain curve, or is it that we should also have a bonds section in our ReaxFF file since the epoxy is crosslinked?

i cannot comment on the science, since i don’t do these kinds of studies. but typical questions i would ask myself are

  • is this a suitable ReaxFF parameterization? ReaxFF parameters are not very transferable, contrary to popular belief.
  • is this result independent from system size and system geometry?
  • is the system properly equilibrated/relaxed before starting the stress-strain analysis?

axel.