[CP2K:1898] Re: Langevin references

[Ondrej Marsalek]

Hi,

this is not what I understood from the papers, the message on this
mailing list from Matthias or from the source code, therefore please
correct me if I am wrong in the following.

Gamma and noisy_gamma get used for the generation of the white noise,
as seen from this source line:

simpar_methods.F:83:       simpar%var_w =
2.0_dp*simpar%temp_ext*simpar%dt*(simpar%gamma+simpar%noisy_gamma)

which corresponds to the RHS in [Kuhne2007].

Shadow_gamma, on the other hand, seems to be added to the Langevin
gamma only for dissipation, as illustrated by the line

integrator.F:164:    gam = simpar%gamma+simpar%shadow_gamma

because the variable gam is the one later used in the propagator. It
is not clear to me what is the intended purpose of shadow_gamma, but
it looks like it could be used to introduce artificial drag into the
system, for example for the purpose of testing the whole procedure of
propagation with noisy forces.

Is this correct or am I reading the code incorrectly?

Best,
Ondrej


On Wed, Mar 18, 2009 at 15:54, Fawzi Mohamed <fa... at gmx.ch> wrote:
>
>
> On 18-mar-09, at 15:50, Daniel_M wrote:
>
>>
>> Hi all,
>>
>> I am doing some MD calculations employing the Langevin Dynamics and I
>> don't know what is exactly the role of the parameter SHADOW_GAMMA. I
>> have been reading the ref. [Kuhne2007] but I understand that in this
>> ref. only the GAMMA and NOISY_GAMMA are mentioned (as gamma_L and
>> gamma_D respectively). Could anybody give me some more references
>> about this?
>
> Shadow gamma is the dissipation that the approximate propagation
> introduces
> You have to evaluate it by doing a test run.
>
> If you are interested in doing a normal langevin dynamics then you
> don't need shadow_gamma.
>
> Fawzi
>>
>>
>> Thanks a lot!
>>
>> Daniel
>>
>
> >
>