Construction of CASCI-type wave functions for very large active spaces

TL;DR

This paper introduces a method to construct CASCI-type wave functions for large active spaces using DMRG, enabling detailed electronic structure analysis and serving as a reference for multi-reference CI calculations.

Contribution

The authors develop a procedure to generate configuration-interaction expansions from DMRG wave functions for very large active spaces, combining Monte Carlo sampling with tensor network states.

Findings

Provides qualitatively correct electronic structure descriptions

Enables analysis of DMRG wave function convergence

Serves as a balanced reference for multi-reference CI

Abstract

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.