  We propose a multireference linearized coupled cluster theory using matrix
product states (MPS-LCC) which provides remarkably accurate ground-state
energies, at a computational cost that has the same scaling as multireference
configuration interaction singles and doubles (MRCISD), for a wide variety of
electronic Hamiltonians. These range from first-row dimers at equilibrium and
stretched geometries, to highly multireference systems such as the chromium
dimer and lattice models such as periodic two-dimensional 1-band and 3-band
Hubbard models. The MPS-LCC theory shows a speed up of several orders of
magnitude over the usual DMRG algorithm while delivering energies in excellent
agreement with converged DMRG calculations. Also, in all the benchmark
calculations presented here MPS-LCC outperformed the commonly used
multi-reference quantum chemistry methods in some cases giving energies in
excess of an order of magnitude more accurate. As a size-extensive method that
can treat large active spaces, MPS-LCC opens up the use of multireference
quantum chemical techniques in strongly-correlated \emph{ab-initio}
Hamiltonians, including two and three-dimensional solids.
