Hi every one:

With the Mesocite DPD, I construct a water bead mesoscopic model (100*100*100) with coarse grain mapping "3 to 1", and set the dissipative coefficient aij=78, the mass of the water bead is 54 amu, and its length is 6.46 Å. others parameter use the software default value.

After a long time DPD running (t=1 ns), I calculate the self-diffusion coefficient of water bead by following Einstein formula, and get its value equal to 207.0333*10^-10 m2/s, which is almost 10 times of experimental value of 23*10^-10 m2/s or others all atomical MD simulation results. And in another DPD running I enable the shearing operation, and get the viscosity of water bead is 0.07136 cP, which is aboult 1/10 of experiment value (0.896 cP).

In order to check my simulation operation, I calculate the same water box with Shinoda 2007 force field in MS6.0, and get the self-diffusion coefficient of water bead equal to 19.15667*10^-10 m2/s, which is comparable with experimental value.By the way, with the MS Martini force field, the self-diffusion coefficient of water bead is 24.67333*10^-10 m2/s, much more close to experimental value.

So the problem that make me confused are these:

1. Why the same mesoscopic water model operated with DPD and Dynamic (using Shinoda 2007 and MS Martini force field) can get so different diffusion coefficient and viscosity, almost one order magnitude difference?
2. If no problem with my DPD simulation operation, then how to explain the self-diffusion coefficient of water bead, can it be connected with experimental value or MD result, if so, how to do it?
3. Refer to Groot's paper (Groot, R. D., and Rabone, K. L. 2001. Biophysical Journal, Vol. 81, No. 2, pp. 725-736.), it seems that the self-diffusion constant of water bead can be connected with water molecule value in MD simulation. As they said "Because the mean square displacement of the water beads is one-third of that of the water molecule, the diffusion constant of the beads is one-third of that of water (coarse grain mapping "3 to 1")" [Page 728]. If it's rigth, multiplied the self-diffusion coefficient of water bead in DPD by 3 will get a much higher value than the MD or experimental results, it is still unreasonable.

4. Change the mapping tactic to  "1 to 1", that one DPD bead represent one water molecule, others parameter set as (aij=25, m=18 amu, rc=4.481Å). The self-diffusion constant of water bead equal to 502.2333 *10^-10 m2/s, is still not comparable with experimental value.

5. If I need to explain the different diffusion capability of water bead in others system, for example the bulk water and the confined water in clay mineral interlayer. Whether it means that I can only make a comparison between the relative values but not the absolute diffusion value of water bead.

Thank you ver much!

Lianfei Kuang