# CheMPS2: Improved DMRG-SCF routine and correlation functions

## Description

This program has been imported from the CPC Program Library held at Queen's University Belfast (1969-2018) Abstract CheMPS2, our spin-adapted implementation of the density matrix renormalization group (DMRG) for ab initio quantum chemistry (Wouters et al., 2014), has several new features. A speed-up of the augmented Hessian Newton–Raphson DMRG self-consistent field (DMRG-SCF) routine is achieved with the direct inversion of the iterative subspace (DIIS). For extended molecules, the active space orbitals can be localized by maximizing the Edmiston–Ruedenberg cost function. These localized orbitals can be or... Title of program: CheMPS2 Catalogue Id: AESE_v2_0 Nature of problem The many-body Hilbert space grows exponentially with the number of single-particle states. Exact diagonalization solvers can therefore only handle small active spaces, of up to 16 electrons in 16 orbitals. Interesting active spaces are often significantly larger. Versions of this program held in the CPC repository in Mendeley Data AESE_v1_0; CheMPS2; 10.1016/j.cpc.2014.01.019 AESE_v2_0; CheMPS2; 10.1016/j.cpc.2015.01.007