# Sorting out viscosity methods

Dear LAMMPS community,

I have read the How to section of the documentation on how to calculate viscosity: http://lammps.sandia.gov/doc/Section_howto.html#howto-21.

It says that there are four ways to calculate viscosity:

1. Lees-Edwards (via fix_deform) + nvt/sllod (using a Nose-Hoover thermostat)

2. Use two frozen walls in relative motion to shear a fluid in between (fix_thermostat+velocity+fix_setforce…)

3. rNEMD with fix_viscosity

4. Equilibrium calculation suing GK.

My question is: can fix_deform and a thermostat be applied to get Lees-Edwards boundary conditions without a wall ( a combination of 1 and 2 somehow). I am looking for example at the following paper by Cao and Likhtman (10.1103/PhysRevLett.108.028302 http://centaur.reading.ac.uk/29426/ ). They used two methods:

1. SLLOD equations with a Langevin thermostat: “When using the Langevin thermostat method, we impose a uniform velocity profile by using peculiar velocities of the particles in the SLLOD equations of motion…”

2. A method using a DPD thermostat: “A second method involves a dissipative particle dynamics (DPD) thermostat [22,23] where the friction is calculated in respect to the velocities of surrounding particles, and thus the velocity profile is allowed to evolve. The shear in this case is driven by the boundary conditions only.”

It is not clear to me if I can implement their approaches in lammps.

Method 1: Involves SLLOD with Langevin thermostat. This is currently not possible in Lammps since nvt/sllod uses Nose-Hoover. Correct?

Method 2: Maybe this is possible using fix_deform + pair_style dpd/tstat. Then the shear will only be applied by fix_deform and not by the equations of motion. It sounds like this would accomplish what they describe in the paper but I should be careful to subtract the streaming velocity from the thermostating. Am I understanding the LAMMPS commands correctly?

Thanks for your assistance,