# Accurate *ab Initio* Spin Densities

journal contribution

posted on 16.12.2015, 21:29 by Katharina Boguslawski, Konrad
H. Marti, Örs Legeza, Markus ReiherWe present an approach for the calculation of spin density
distributions
for molecules that require very large active spaces for a qualitatively
correct description of their electronic structure. Our approach is
based on the density-matrix renormalization group (DMRG) algorithm
to calculate the spin density matrix elements as a basic quantity
for the spatially resolved spin density distribution. The spin density
matrix elements are directly determined from the second-quantized
elementary operators optimized by the DMRG algorithm. As an analytic
convergence criterion for the spin density distribution, we employ
our recently developed sampling-reconstruction scheme [

*J. Chem. Phys.***2011**,*134*, 224101] to build an accurate complete-active-space configuration-interaction (CASCI) wave function from the optimized matrix product states. The spin density matrix elements can then also be determined as an expectation value employing the reconstructed wave function expansion. Furthermore, the explicit reconstruction of a CASCI-type wave function provides insight into chemically interesting features of the molecule under study such as the distribution of α and β electrons in terms of Slater determinants, CI coefficients, and natural orbitals. The methodology is applied to an iron nitrosyl complex which we have identified as a challenging system for standard approaches [*J. Chem. Theory Comput.***2011**,*7*, 2740].