# fix langevin

I compute every particle’s total force using group function fcm for every timestep, and the total force should include drag force, random force and the repulsive force between the particles and polymer network’s atoms if particles diffusion in polymer networks.

I compute every particle's total force using group function fcm for every
timestep, and the total force should include drag force, random force and
the repulsive force between the particles and polymer network's atoms if
particles diffusion in polymer networks.

​unless you have a force term​ adding a drift to your system, the sum of
all forces (aka the force on the center of mass) *should* be (on average)
zero and the langevin thermostat (or any other regular thermostat) should
not change that.

you are comparing apples and oranges when you compare this term to the MSD.

axel.

thank you for your reply. I understand you are talking about the total force of the whole system. But I focus on the single particle which can diffuse freely or diffuse in a polymer network. The polymer network will hinder the particle diffuse, so diffusive particles’ MSD (statistic from many particles) will different between with and without polymer network. Force is reason for changing the state of motion of the diffusive particle. So I compute the single diffusive particle’s total force (for statistic), and try to find their difference in the case of with and without polymer network. But the results obtained from compute diffusive particles are no difference, which confused me.

force of the whole system. But I focus on the single particle which can
diffuse freely or diffuse in a polymer network. The polymer network will
hinder the particle diffuse, so diffusive particles' MSD (statistic from
many particles) will different between with and without polymer network.
Force is reason for changing the state of motion of the diffusive
particle. So I compute the single diffusive particle's total force (for
statistic), and try to find their difference in the case of with and
without polymer network. But the results obtained from compute diffusive
particles are no difference, which confused me.

​again, if there is no drift imposed, e.g. through a fix or an
inhomogeneity of the system, the average of the force should be 0.
regardless of how many particles are in the group​. the impact of fix
langevin should not change that. the einstein relation after which the
(self-)diffusion constant is computed from the MSD is based on such a
random walk.
let's consider the most extreme case of hard spheres (force is either 0 or
infinity): here the diffusivity is obviously not determined by the force,
but rather by the average time or distance between two collisions. so i
contend, that your interpretation of what determines the magnitude of
diffusion is not correct.

axel.