If $V_\lambda$, $V_\mu$ and $V_\nu$ are irreducible representations of $\operatorname{GL}_n$, the Littlewood-Richardson coefficient $c_{\lambda\mu}^\nu$ denotes the multiplicity of $V_\nu$ in the direct sum decomposition of the tensor product of $V_\lambda$ and $V_\nu$. Knutson and Tao (JAMS 1999, link at AMS site) proposed a "hive model" for Littlewood-Richardson coefficients.
Is there an analogous model for such tensor product multiplicities for Lie groups of types B, C or D?
The recent work of Goncharov-Shen gives a good generalization of the hive model for any reductive group $ G$. They show that $n$-fold tensor product multiplicities for $ G $ are counted by positive tropical integral points of the space $ G^\vee \setminus ( G^\vee / N)^n $. When $ G = GL_m$ and $n = 3$, this gives the Hive model. When $ G = GL_m $ and $ n = 4 $, this gives the octahedron recurrence.
Unfortunately, outside of type $ A$, it is hard to give a simple description of their set of positive tropical points. To do so requires some choices, and once one makes these choices, you end up with one of the Berenstein-Zelevinsky models which Allen mentioned.
There are conjectural ones in the Berenstein-Zelevinsky paper referenced in that one. They have another paper with a general theorem, Tensor product multiplicities, canonical bases and totally positive varieties (Inventiones mathematicae 143 (2001) pp 77–128, https://doi.org/10.1007/s002220000102), that gives (many) polyhedral models for any Lie type.
There is some recent progress on this question. JiaRui Fei https://arxiv.org/pdf/1603.02521.pdf has given a generalization of the hive model for all semisimple Lie algebras, based on the theory of cluster algebras. In type A, his model specializes to the Knutson-Tao hive model.
