Three-Legged Tree Tensor Networks with SU(2) and Molecular Point Group Symmetry

We extend the three-legged tree tensor network state (T3NS) [J.  Chem. Theory Comput. 2018, 14, 2026-2033] by including spin and the real abelian point group symmetries.  T3NS intersperses physical tensors with branching tensors.  Physical tensors have one physical index and at most two virtual indices.  Branching tensors have up to three virtual indices and no physical index. In this way, T3NS combines the low computational cost of matrix product states and their simplicity for implementing symmetries, with the better entanglement representation of tree tensor networks. By including spin and point group symmetries, more accurate calculations can be obtained with lower computational effort. We illustrate this by presenting calculations on the bis($\mu$-oxo) and $\mu-\eta^2:\eta^2$ peroxo isomers of $[\mathrm{Cu}_2\mathrm{O}_2]^{2+}$. The used implementation is available on github.