[lammps-users] about the units in DPD

Hi, all user

In DPD, I would like to use the REAL units.
In the " pair_coeff" command, 3 parameters are needed: a, gamma, cutoff.
Now i am confusing on the gamma number with REAL units. Generally in, LJ units, we choose gamma=4.5 (if we set sigma=3). I want to know what the number of gamma is, if it is with REAL units, suppose we know all other necessary parameters, such as mass=20grams/mol, temperature=300K …


The doc page for pair dpd gives the eq (and the paper) and
the units for gamma = force/velocity, which is fmsec-Kcal/mole
in real units. You'll have to think carefully about how
to convert from LJ units, but the physical interpretation
of gamma should be clear.

The way to test if you've done it right is as follows:
You can create 2 input scripts in different units,
say in.dpd.lj and in.dpd.real, that do the same thing.
If you run them (and convert
the thermo output from LJ <-> real), you will get
the exact same answer on every timestep to as high
a precision as you are careful to do the conversion.
Of course after many timesteps the 2 systems will
slowly diverge due to round-off.


2010/8/26 Trojan 007 <[email protected]>: