[BUG] Implicit Species in Discontinuous

[tez-orr123]

Describe the bug
When running a discontinuous problem with an implicit species, I run into an issue where the trapped species has no solution attribute.

To Reproduce

import festim as F
import numpy as np

tungsten = F.Material(
    D_0=1,
    E_D=0, 
    K_S_0=1,
    E_K_S=0,
)

# Subdomains
W_volume = F.VolumeSubdomain1D(id=6, borders=[0, 1], material=tungsten)

# Problem
my_model = F.HydrogenTransportProblemDiscontinuous()
my_model.mesh = F.Mesh1D(np.linspace(0, 1, 100))

my_model.subdomains =  [
    W_volume,
]

Deuterium = F.Species("D", subdomains=my_model.volume_subdomains)
trapped_T = F.Species("T_trapped", mobile=False, subdomains=my_model.volume_subdomains)

empty_traps = F.ImplicitSpecies(n=1, others=[trapped_T], name='implicit_species')

my_model.species = [trapped_T, Deuterium]

my_model.reactions = [
    F.Reaction(
        reactant=[empty_traps, Deuterium],
        product=[],
        k_0=1,
        E_k=0,
        volume=W_volume,
    ),
]

my_model.temperature = 300

my_model.settings = F.Settings(
    transient=False,
    atol=1e-8,
    rtol=1e-10,
)

my_model.initialise()

Produces:

Traceback (most recent call last):
  File "/home/tezorr/festim-fellowship/my_monoblock_case/mwe_2.py", line 47, in <module>
    my_model.initialise()
    ~~~~~~~~~~~~~~~~~~~^^
  File "/home/tezorr/miniconda3/envs/festim-fellowship/lib/python3.14/site-packages/festim/hydrogen_transport_problem.py", line 1174, in initialise
    self.create_subdomain_formulation(subdomain)
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^
  File "/home/tezorr/miniconda3/envs/festim-fellowship/lib/python3.14/site-packages/festim/hydrogen_transport_problem.py", line 1442, in create_subdomain_formulation
    reaction.reaction_term(self.temperature_fenics, reactant_concentrations=reactant_concentrations)
    ~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/tezorr/miniconda3/envs/festim-fellowship/lib/python3.14/site-packages/festim/reaction.py", line 177, in reaction_term
    reactant_concentrations[i] = reactant.concentration
                                 ^^^^^^^^^^^^^^^^^^^^^^
  File "/home/tezorr/miniconda3/envs/festim-fellowship/lib/python3.14/site-packages/festim/species.py", line 171, in concentration
    raise ValueError(
    ...<2 lines>...
    )
ValueError: Cannot compute concentration of implicit_species because T_trapped has no solution.

[tez-orr123]

For some reason, this is one of the tests and it works

import festim as F
import numpy as np

my_model = F.HydrogenTransportProblemDiscontinuous()

N = 1500
vertices = np.linspace(0, 1, num=N)
my_model.mesh = F.Mesh1D(vertices)

material = F.Material(D_0=2.0, E_D=0)

material.K_S_0 = 2.0
material.E_K_S = 0

subdomain = F.VolumeSubdomain1D(
    1, borders=[vertices[0], vertices[-1]], material=material
)

left_surface = F.SurfaceSubdomain1D(id=1, x=vertices[0])
right_surface = F.SurfaceSubdomain1D(id=2, x=vertices[-1])

my_model.subdomains = [subdomain, left_surface, right_surface]
my_model.surface_to_volume = {
    right_surface: subdomain,
    left_surface: subdomain,
}

H = F.Species("H", mobile=True)
trapped_H = F.Species("H_trapped", mobile=False)
empty_trap = F.ImplicitSpecies(n=0.5, others=[trapped_H])

my_model.species = [H, trapped_H]

for species in my_model.species:
    species.subdomains = my_model.volume_subdomains

my_model.reactions = [
    F.Reaction(
        reactant=[H, empty_trap],
        product=[trapped_H],
        k_0=2,
        E_k=0,
        p_0=0.1,
        E_p=0,
        volume=domain,
    )
    for domain in my_model.volume_subdomains
]

my_model.boundary_conditions = [
    F.FixedConcentrationBC(left_surface, value=0.05, species=H),
    F.FixedConcentrationBC(right_surface, value=0.2, species=H),
]

my_model.temperature = 300

my_model.settings = F.Settings(atol=1e-5, rtol=1e-5, transient=False)


my_model.initialise()
my_model.run()

[tez-orr123]

This is the fix in HydorgenTransportProblemDiscontinuous.create_subdomain_formulation all we have to do is extend the hack to the others attribute of the implicit species

            # check if we have implicit species:
            for reactant in reaction.reactant:
                if isinstance(reactant, festim.species.ImplicitSpecies):
                    for other_spe in reactant.others:
                        if other_spe not in list_of_species_to_override:
                            list_of_species_to_override.append(other_spe)

            for species in list_of_species_to_override:
                if isinstance(species, festim.species.Species):
                    # TODO remove
                    # temporarily overide the solution to the one of the subdomain
                    species.solution = species.subdomain_to_solution[reaction.volume]

[RemDelaporteMathurin]

Current workaround is to replace the implicit species by an explicit one