CheMPS2 is a scientific library which contains a spin-adapted implementation
of the density matrix renormalization group (DMRG) for ab initio quantum
chemistry. This wavefunction method allows to obtain numerical accuracy in
active spaces beyond the capabilities of full configuration interaction (FCI),
and allows to extract the 2-, 3-, and 4-particle reduced density matrices
(2-, 3- and 4-RDM) of the active space.

For general active spaces up to 40 electrons in 40 orbitals can be handled with
DMRG, and for one-dimensional active spaces up to 100 electrons in 100 orbitals.
The 2-RDM of these active spaces can also be easily extracted, while the 3- and
4-RDM are limited to about 28 orbitals.