| PackageDescription | Documentation of the libchemps2-3 package
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.
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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.
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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.
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