  We present a procedure to construct a configuration-interaction expansion
containing arbitrary excitations from an underlying
full-configuration-interaction-type wave function defined for a very large
active space. Our procedure is based on the density-matrix renormalization
group (DMRG) algorithm that provides the necessary information in terms of the
eigenstates of the reduced density matrices to calculate the coefficient of any
basis state in the many-particle Hilbert space. Since the dimension of the
Hilbert space scales binomially with the size of the active space, a
sophisticated Monte Carlo sampling routine is employed. This sampling algorithm
can also construct such configuration-interaction-type wave functions from any
other type of tensor network states. The configuration-interaction information
obtained serves several purposes. It yields a qualitatively correct description
of the molecule's electronic structure, it allows us to analyze DMRG wave
functions converged for the same molecular system but with different parameter
sets (e.g., different numbers of active-system (block) states), and it can be
considered a balanced reference for the application of a subsequent standard
multi-reference configuration-interaction method.
