1303.0616 (Yingjin Ma et al.)

Assessing the efficiency of various natural orbitals as the basis of
large active space density matrix renormalization group calculations    [PDF]

Yingjin Ma, Haibo Ma

Recently proposed density matrix renormalization group complete active space self consistent field (DMRG-CASSCF) method provided a promising multi-reference tool for treating large complicated electron correlation problems because it can handle up to 40 active orbitals for general systems. However, the convergence difficulties and the expensive computational costs for large active spaces in orbital optimization iterations as well as the complexity in selecting active orbitals still greatly limit the applications of these promising methods. In this paper, we assess the efficiency of using natural orbitals as the basis of large active space DMRG calculations, which include the traditional NOs obtained by MP2, CISD and CASPT2 calculations, and NOs acquired from preliminary moderate DMRG calculations (e.g. preserved states less than 500), as well as Kohn-Sham orbitals, Hartree Fock canonical orbitals and CASSCF optimized orbitals. The tested systems include N$_2$, transition metal Cr$_2$ and one-dimensional hydrogen polyradical chain systems under both equilibrium and dissociation situations. The results of DMRG-complete active space configuration interaction (DMRG-CASCI) calculations with these orbitals are carefully compared with those by DMRG-CASSCF calculations. It is shown that DMRG-CASCI calculation in a basis of carefully chosen NOs can provide a less expensive alternative to standard DMRG-CASSCF calculation and this can also avoid the convergence difficulties of orbital optimization for large active space.

View original: http://arxiv.org/abs/1303.0616