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Re: [ESPResSo-users] Soft sphere interaction

From: Axel Arnold
Subject: Re: [ESPResSo-users] Soft sphere interaction
Date: Fri, 31 Oct 2014 21:59:54 +0100

On 31.10.2014, at 20:37, Xikai Jiang <address@hidden> wrote:


For the cut-off problem, have you check your "skin" length? Since the total cut-off = "maximum cut-off you set in the command" + "length of skin". The skin length is set by "setmd skin xx”.

No, that is not correct. The total cutoff for the potential is just r_cut + r_offset. What you refer to is the Verlet cutoff radius, which is r_cut + r_offset + skin, but has no influence on computed observables, just on the computer time it takes to compute them.


Date: Fri, 31 Oct 2014 11:39:23 +0000
From: address@hidden
To: address@hidden
Subject: [ESPResSo-users] Soft sphere interaction

Dear all,

I have a question about soft spheres interaction. I tried to use one to get a potential of interactions, corresponding to linear growth of forces while the distance goes to zero (to avoid overlapping on small distances, for LJ I initially had sigma=0.3 and sigma =0.6)
So I took a formula for potential
U(r)=169.37*(r-0.65)^2, which corresponds to the 2nd virial coefficient I'm trying to reproduce, and chose the parameters for SS Potential
a = 169.37, r_off = 0.65 and finally n = -2.
Probably the last parameter cannot be chosen like that(negative), because I got energy of interaction exactly the same as I expected, but the forces are equal to zero in the whole range of distances. I had a look at the code, but didn't find what the issue can be related with. By the way, there is another problem with potential - it is being cut on 0.9 in case of r_off=0.65. If r_off=0.4, the cut is on 0.65. I don't really understand where this shift of 0.25 comes from.

Apart from that, could you please tell me if I can use both dpd_interactions between particular types of particles in combination with Lattice-Boltzmann Method?
Thanks in advance!


Kind regards,
Aleksei Kabedev

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