[lammps-users] sigma value in DPD

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1

Hello,

I’d like to use my own sigma value in pair_style DPD.
Currently, Sigma is automatically set equal to sqrt(2 Kb T gamma).
Do anyone know how to?

Thank you.

2

Hello,
I'd like to use my own sigma value in pair_style DPD.
Currently, Sigma is automatically set equal to sqrt(2 Kb T gamma).
Do anyone know how to?

have you considered: vi pair_dpd.cpp ?

axel.

3

Hello,
I'd like to use my own sigma value in pair_style DPD.
Currently, Sigma is automatically set equal to sqrt(2 Kb T gamma).
Do anyone know how to?

have you considered: vi pair_dpd.cpp ?

also, if you review the definition of sigma carefully, then it should
be obvious that changing sigma is equivalent to changing the
temperature parameter T.

so if you absolutely want to set sigma explicitly,
why not invert the formula and define an equal style
variable that computes the value of T required from
the sigma that you desire?

cheers,
    axel.

4

Thank you.

in line 277 of pair_dpd.cpp file,
sigma value is defined as the equation “sqrt(2 Kb T gamma)”.
So, I will need to change many things to be able to set my own sigma value in LAMMPS script.
e.g.
pair_style dpd T cutoff seed

pair_coeff type1 type2 A gamma SIGMA cutoff



Do you have any idea on what I need to change in the pair_dpd.cpp file?
5

Many pair styles have per-type params like LJ for sigma and epsilon.
So you could look at one of those and see how they allocated and
maintain per-type arrays - e,g. pair_lj_cut.cpp

Steve

6

Why do you want to do this?

sigma = sqrt(2kbT*gamma) is the fluctuation dissipation theorem.

You shouldn’t arbitrarily choose a sigma and gamma.