## Section: Application Domains

### Chemistry

The treatment of *chemical reactions* in the framework of OT is a rather recent development.
The classical theory must be extended to deal with
the transfer of mass between different particle species by means of chemical reactions.
That extension is still far from complete at the moment,
but there is a lot of progress currently, some of which we try to capture in the workshop.

A promising and significant recent advance is the introduction and analysis of a novel metric
that combines the pure transport elements of the Wasserstein distance
with the annihilation and creation of mass, which is a first approximation of chemical reactions.
The logical next challenge is the extension of OT concepts to vectorial quantities,
which allows to rewrite cross-diffusion systems for the concentration of several chemical species as gradient flows in the associated metric.
An example of application is the modeling of a *chemical vapor deposition process*,
used for the manufacturing of thin-film solar cells for instance.
This leads to a degenerate cross-diffusion equations, whose analysis — without the use of OT theory — is delicate.
Finding an appropriate OT framework to give the formal gradient flow structure a rigorous meaning
would be a significant advance for the applicability of the theory, also in other contexts, like for biological multi-species diffusion.

A very different application of OT in chemistry is a novel approach to the understanding of *density functional theory* (DFT)
by using optimal transport with “Coulomb costs”, which is highly non convex and singular.
Albeit this theory shares some properties with the usual optimal transportation problems,
it does not induce a metric between probability measures.
It also uses the multi-marginal extension of OT, which is an active field on its own right.