Spin-adapted DMRG for ab initio quantum chemistry

 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 one
 to obtain numerical accuracy in active spaces beyond the capabilities
 of full configuration interaction (FCI), and allows one 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.
 .
 When the active space size becomes prohibitively expensive for FCI,
 DMRG can be used to replace the FCI solver in the complete active
 space self consistent field (CASSCF) method and the corresponding
 complete active space second order perturbation theory (CASPT2).
 The corresponding methods are called DMRG-SCF and DMRG-CASPT2,
 respectively. For DMRG-SCF the active space 2-RDM is required, and
 for DMRG-CASPT2 the active space 4-RDM.