Re: [ESPResSo-users] Espressomd-users Digest, Vol 83, Issue 4

[Nairhita Samanta]

Hello everyone!

Am I making any mistake assuming that the mass of each particle in Espresso is inherently fixed irrespective of the size of the particles. The gamma should be size-dependent instead.

On Wed, Sep 20, 2017 at 5:00 PM, <address@hidden> wrote:

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Today's Topics:

   1. Re: Choice of timestep and langevin-gamma (Georg Rempfer)
   2. Re: Choice of timestep and langevin-gamma
      (address@hidden)


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Message: 1
Date: Tue, 19 Sep 2017 21:42:44 +0200
From: Georg Rempfer <address@hidden>
To: "address@hidden" <address@hidden>
Subject: Re: [ESPResSo-users] Choice of timestep and langevin-gamma
Message-ID:
        <CAA6P0aKkgh15M8Dr_z0vh75mrsEWhP9FuKiSwRsqhTDWkPmaddress@hidden>
Content-Type: text/plain; charset="utf-8"

There is a feature called LANGEVIN_PER_PARTICLE, that allows you to set
different gammas for different particles.

On Tue, Sep 19, 2017 at 4:38 PM, Clemens Jochum <address@hidden
> wrote:

> Dear all,
>
> I just noticed a mistakeI made. In the user-guide it is stated that
> gamma is not the inverse relaxation time tau anymore, but it is now
> defined as mass / tau. In this case if I want to achieve tau^-1 = 1.5
> ps^-1 for a particle of mass2 = 660, I have to set gamma = 100, which
> seems high.
>
> At the same time the other particles with mass1 = 20 will have tau^-1 =
> 50 ps^-1 (mass2 / mass1 = 33). But with the way the Langevin-thermostat
> is implemented in ESPResSo it is not possible to prevent this large
> difference between the two particle types with different masses.
>
> Could this lead to some problems? Or is there a way to assign a fixed
> value to tau and tau^-1 for all masses in ESPResSo?
>
> Best,
> Clemens
>
> > The other parameter I am concerned about is gamma of the
> > langevin-thermostat. So far I just used gamma = 1. = 10 ps^-1, which as
> > I recently read is a little bit high. In my system I simulate a charged
> > macromolecule with counter-ions in an implicit solvent (water). Is a
> > value of gamma = 0.2 = 2 ps^-1 a good fit?
>
>
> On 19.09.2017 13:11, Clemens Jochum wrote:
> > Dear all,
> >
> > I read previously that for a system with Temp = 1. and mass = 1. a good
> > value for the timestep is 0.01. I have Temp  = 2.5 and mass1 = 20 and
> > mass2 = 660. Is it correct to assume that I can scale the timestep
> > inversely to the thermal velocity with a factor of (m / kT)^(1/2) ~ 3
> > (using the mass1)? I have been using a timestep of 0.05 = 5e-15 s so far
> > without any major problems.
> >
> > The other parameter I am concerned about is gamma of the
> > langevin-thermostat. So far I just used gamma = 1. = 10 ps^-1, which as
> > I recently read is a little bit high. In my system I simulate a charged
> > macromolecule with counter-ions in an implicit solvent (water). Is a
> > value of gamma = 0.2 = 2 ps^-1 a good fit?
> >
> > Best,
> > Clemens
> >
> >
> >
> >
> >
> >
>
>
>
>
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Message: 2
Date: Tue, 19 Sep 2017 23:16:51 +0200
From: "address@hidden" <address@hidden>
To: <address@hidden>
Subject: Re: [ESPResSo-users] Choice of timestep and langevin-gamma
Message-ID: <afb63af5-1462-9ef7-a585-address@hidden>
Content-Type: text/plain; charset="utf-8"; format=flowed

Thanks! That looks like what I need.
Best,
Clemens

On 09/19/2017 09:42 PM, Georg Rempfer wrote:
> There is a feature called LANGEVIN_PER_PARTICLE, that allows you to set
> different gammas for different particles.
>
> On Tue, Sep 19, 2017 at 4:38 PM, Clemens Jochum
> <address@hidden <mailto:address@hiddenac.at>> wrote:
>
>     Dear all,
>
>     I just noticed a mistakeI made. In the user-guide it is stated that
>     gamma is not the inverse relaxation time tau anymore, but it is now
>     defined as mass / tau. In this case if I want to achieve tau^-1 = 1.5
>     ps^-1 for a particle of mass2 = 660, I have to set gamma = 100, which
>     seems high.
>
>     At the same time the other particles with mass1 = 20 will have tau^-1 =
>     50 ps^-1 (mass2 / mass1 = 33). But with the way the Langevin-thermostat
>     is implemented in ESPResSo it is not possible to prevent this large
>     difference between the two particle types with different masses.
>
>     Could this lead to some problems? Or is there a way to assign a fixed
>     value to tau and tau^-1 for all masses in ESPResSo?
>
>     Best,
>     Clemens
>
>     > The other parameter I am concerned about is gamma of the
>     > langevin-thermostat. So far I just used gamma = 1. = 10 ps^-1, which as
>     > I recently read is a little bit high. In my system I simulate a charged
>     > macromolecule with counter-ions in an implicit solvent (water). Is a
>     > value of gamma = 0.2 = 2 ps^-1 a good fit?
>
>
>     On 19.09.2017 13:11, Clemens Jochum wrote:
>     > Dear all,
>     >
>     > I read previously that for a system with Temp = 1. and mass = 1. a
>     good
>     > value for the timestep is 0.01. I have Temp  = 2.5 and mass1 = 20 and
>     > mass2 = 660. Is it correct to assume that I can scale the timestep
>     > inversely to the thermal velocity with a factor of (m / kT)^(1/2) ~ 3
>     > (using the mass1)? I have been using a timestep of 0.05 = 5e-15 s
>     so far
>     > without any major problems.
>     >
>     > The other parameter I am concerned about is gamma of the
>     > langevin-thermostat. So far I just used gamma = 1. = 10 ps^-1,
>     which as
>     > I recently read is a little bit high. In my system I simulate a
>     charged
>     > macromolecule with counter-ions in an implicit solvent (water). Is a
>     > value of gamma = 0.2 = 2 ps^-1 a good fit?
>     >
>     > Best,
>     > Clemens
>     >
>     >
>     >
>     >
>     >
>     >
>
>
>
>



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