Low-Cost Generation of Optimal Molecular Orbitals for Multireference CI Expansion: Natural Orbitals Versus Rényi Entropy Minimized Orbitals Provided by the Density Matrix Renormalization Group

The aim of this work is to bring novel concepts on orbital optimization protocols employed in the DMRGDensity matrix renormalization group procedure closer to the quantum chemistry community. While a more rigorous mathematical approach has been presented previously in Phys. Rev. Lett. 117, 210402, here we focus on technical aspects to obtain natural orbitalNatural orbitals and Rényi entropyRényi entropy based orbital optimization in practice via low cost DMRGDensity matrix renormalization group calculations. We also illustrate that the use of these orbital sets can increase the accuracy of large scale DMRGDensity matrix renormalization group calculations, i.e., simulations with large bond dimension, relative to the FCI limit. A pedagogical analysis is presented via the bond stretching of the N \(_2\) molecule.