Hi Chen Chen:

You should use a hybrid atom style in lammps:

atom_style hybrid bond dpd

It means some particles in dpd simulation connected by bonds (polymer).

The most important thing is to choose a suitable bond style.

If you want your bond style to consist with the settings in Groot and Madden’s, you can use the harmonic style.

bond_style harmonic

In Groot and Madden’s [J. Chem. Phys.,108,8713,(1998)] paper, as you are saying, the spring force is set as:

f=C__*r__ij
while in my script it is:
f=C*(r-ro)

Therefore, in my script, it should be r0=0.

In lammps user guide, the equation is given by spring energy:

E=K(r-r0)^2

Here, K=

So if C= my simulation, the K in lammps is 2.0.

If it has three types of the bonds in my system, the following commands will be important:

bond_coeff 1 2.0 0

bond_coeff 2 2.0 0

bond_coeff 3 2.0 0

Here, K=2.0, r0=0

But something should be noticed.

In J. Chem. Phys.,107,4423,(1997)[Groot], “the spring constant is chosen such that the average distance between connected particles coincides with the point where the pair correlation function has its maximum, see Fig. 2”.

Please see Fig.2 of this paper, it can be easily concluded:

Where r is within 0.85-0.875, the pair correlation function g® gets the maximum.

Therefore, in Groot’s papers, the actual equilibrium bond distance is within 0.85-0.875.

In my script, I used the following commands in simulation.

bond_coeff 1 2.0 0.875

bond_coeff 2 2.0 0.875

bond_coeff 3 2.0 0.875

I did many experiments, I found r0 (0.0 or 0.875) just has slight influence on the final results.

In my opinion, r0=0.875 is recommended.

Regards

Tao Zhou

2009-07-09