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.