Rationale for the Extrapolation Procedure in Selected Configuration Interaction

Understand

Selected configuration interaction (SCI) methods have emerged as state-of-the-art methodologies for achieving high accuracy and generating benchmark reference data for ground and excited states in small molecular systems.

• However, their precision relies heavily on extrapolation procedures to produce a final estimate of the exact result.
• Using the structure of the exact electronic energy landscape, we provide a rationale for the common linear extrapolation of the variational energy as a function of the second-order perturbative correction.
• In particular, we demonstrate that the energy gap and the coupling between the so-called internal and external spaces are the key factors determining the rate at which the linear regime is reached.