Dear Othon:

I can't provide a definitive answer-there's not enough information here for that. But here are a few questions to think about:

* How are you measuring diffusion coefficients accurately when the size of your box can adjust, displacing the positions of the water molecules along with it?

* Are you sure that you are handling periodic boundary conditions correctly?

* How many molecules are you using to determine the diffusion coefficent?

From: Moultos, Othon

Sent: Sunday, August 25, 2013 11:48 AM

To: '[email protected]'

Subject: RE: [lammps-users] ewald and pppm give different results for

diffusion of CO2 in water

Thanks for the answer. Well the diffusion is known to have a dependence on

system size. I have checked the case of 5 co2s diffusing in 500, 1000 and

2000 waters. I am calculating the diffusion from the info acquired by the

npt file which contains the velocities, positions and continuation vectors

of the com of molecules. Since I construct the unfolded positions from these

positions it doesnâ€™t matter in which ensemble I perform the simulation. I

calculate both msds and vacfs and I found it strange to have differences

while using ewald and pppm. Am I missing something?

do you have any proof that there is a *systematic* difference between

the two methods?

just getting different results from two trajectories is not

sufficient, it could happen simply from different starting conditions.

since your kspace convergence parameter of 1.0e-4 is rather low,

differences between trajectories (but less so in properly converged

averages) are to be expected.to build quickly.

have you checked for statistical convergence? with so few molecules

obtaining converged results can be difficult.

axel.