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 method allows one to obtain
 numerical accuracy in active spaces beyond the capabilities of
 full configuration interaction (FCI): up to 40 electrons in 40
 orbitals for general active spaces; and up to 100 electrons in
 100 orbitals for one-dimensional active spaces, such as the
 pi-system of all-trans polyenes.
 .
 For an input Hamiltonian and targeted symmetry sector, the library
 performs successive DMRG sweeps according to a user-defined
 convergence scheme. As output, the library returns the minimal
 encountered energy as well as the second order reduced density
 matrix (2-RDM) of the active space. With the latter, various
 molecular properties can be calculated, as well as the gradient and
 Hessian for orbital rotations or nuclear displacements. In addition,
 several correlation functions can be obtained to investigate the
 electronic structure in the active space.
 .
 This version of chemps2 is parallelized for shared memory
 architectures with OpenMP.