Re: Gradient magnetic field

[Jean-Noël Grad]

Hi Kseniia,

This feature doesn't exist yet, but the necessary building blocks are 
available. There is certainly more than one way to implement it, but 
here is how I would go about it. Starting from the FlowField [1], whose 
equation is force = -gamma * (u(p.pos) - p.v), one can substitute unity 
for gamma, p.dip for p.v, and use an appropriate vector product 
expression instead of the subtraction. The position-dependent quantity u 
is a vector field, which in ESPResSo is stored in a class that provides 
a definition for the corresponding Jacobian matrix. Therefore all 
ingredients to compute forces and torques should already be here.

The new C++ kernel can be adapted from the Viscous class [2], and needs 
to return a ParticleForce object, which stores two Vector3d objects for 
the force and torque (right now the Viscous kernel only returns a 
Vector3d for the force). Then one has to implement a script interface 
class adapted from FlowField [3], with a different name and taking as 
template parameter the new kernel class instead of the Viscous class, 
and register it in the initializer list [4]. Finally one has to create 
the corresponding Python class by adapting the FlowField one [5].

If you are familiar with C++ and template programming, and have time to 
develop code, this should be within your reach. The feature can be 
developed in two stages, starting with the C++ kernel and a unit test 
based on the Viscous test [6] to run it with a mock Particle class [7]. 
Then the Python class and script interface can be designed. You could 
join our coding days remotely via Zoom if you need assistance with this 
coding project.

Best,
JN

Links:
[1] 
https://espressomd.github.io/doc/espressomd.html#espressomd.constraints.FlowField
[2] 
https://github.com/espressomd/espresso/blob/7ad0534/src/core/field_coupling/couplings/Viscous.hpp#L36-L40
[3] 
https://github.com/espressomd/espresso/blob/7ad0534/src/script_interface/constraints/initialize.cpp#L46
[4] 
https://github.com/espressomd/espresso/blob/7ad0534/src/script_interface/constraints/initialize.cpp#L57-L58
[5] 
https://github.com/espressomd/espresso/blob/7ad0534/src/python/espressomd/constraints.py#L516-L542
[6] 
https://github.com/espressomd/espresso/blob/7ad0534/src/core/unit_tests/field_coupling_couplings_test.cpp#L99-L117
[7] 
https://github.com/espressomd/espresso/blob/7ad0534/src/core/unit_tests/field_coupling_couplings_test.cpp#L82-L88

On 3/8/24 08:14, Ксения Астахова wrote:
> Hello everyone!
>
> I want to create a gradient magnetic field, but I see that this 
> functionality isn't supported yet. I was wondering if anyone had 
> implemented it or knows how to deal with it.
>
> Best wishes,
> Kseniia