Modified Velocity Verlet for DPD

[Luis Padilla]

Hello,

I am looking for a way to use the modified velocity Verlet algorithm for simulations using DPD. In the documentation it says that the NVE integrator uses the standard velocity Verlet. The thing is that when I try to replicate some results in literature for non-equilibrium self-assembly of diblock-copolymers, the morphologies are not the same as for those results. I'm using exactly the same interaction parameters and initial condition, the only difference is that the published results were obtained using a prefactor to compute an "effective" velocity for the calculation of the force (modified velocity verlet as they call it).

I'm including a link where is shown this modified algorithm, basically that \lambda in the force is the modification that scales the velocity:

https://docs.lammps.org/fix_mvv_dpd.html

Any suggestion is highly appreciated.

Greetings,

Luis Padilla

[Michael Howard]

Hi Luis,

This does not exist in HOOMD, but you could implement it as a new integration method in a plugin.

Regards,

Mike

[Joshua Anderson]

Luis,

As Michael said, you are welcome to implement your own integrator in a plugin.

I would seriously question any simulation results where the thermodynamic state depends on the integrator discretization. If they do, that is a sign that the system is unstable and there are exponential errors in the trajectory. The damping term in DPD and Langevin thermostats can keep systems like this from blowing up completely, but the error is still present.

Well-behaved simulations should produce the same thermodynamic state whether you use DPD, Langevin, NVT, or NPT (even NVE, though it can be tricky to target a specific temperature with NVE) with the same conservative potential. Try another thermostat (or several) with the conservative part of the DPD potential as a cross-check and see which state is the correct one.
------
Joshua A. Anderson, Ph.D.
Research Area Specialist, Chemical Engineering, University of Michigan

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[Luis Padilla]

Thank you both. I will try Langevin's thermostat to see if I get something different.