  We explore various ways to group orbitals into clusters in a matrix product
state (MPS). We explain how a generic cluster MPS can often lead to an increase
in computational cost and instead propose a special cluster structure,
involving only the first and last orbitals/sites, with a wider scope for
computational advantage. This structure is a natural formalism to describe
correlated multireference (MR) theories. We demonstrate the flexibility and
usefulness of this approach by implementing various uncontracted MR
configuration interaction, perturbation and linearized coupled cluster theories
using an MPS with large cluster sites. Applications to the nitrogen dimer, the
chromium dimer, and benzene, including up to triple excitations in the external
space, demonstrate the utility of an MPS with up to two large sites. We use our
results to analyze the quality of different multireference approximations.
