Flexible ansatz for N-body configuration interaction.

[Display omitted] • A unifying framework for multi-determinant wavefunctions is developed. • Mathematical requirements for wavefunction ansätze to be size-consistent are derived. • New wavefunction ansatze are proposed, e.g. the determinant ratio wavefunction form. • Geminal-product wavefunctions are generalized. We present a Flexible Ansatz for N-body Configuration Interaction (FANCI) that includes any multideterminant wavefunction. This ansatz is a generalization of the Configuration Interaction (CI) wavefunction, where the coefficients are replaced by a specified function of certain parameters. By making an appropriate choice for this function, we can reproduce popular wavefunction structures like CI, Coupled-Cluster, Tensor Network States, and geminal-product wavefunctions. The universality of this framework suggests a programming structure that allows for the easy construction and optimization of arbitrary wavefunctions. Here, we will discuss the structures of the FANCI framework and its implications for wavefunction properties, particularly accuracy, cost, and size-consistency. We demonstrate the flexibility of this framework by reconstructing popular wavefunction ansätze and modifying them to construct novel wavefunction forms. FANCI provides a powerful framework for exploring, developing, and testing new wavefunction forms. [ABSTRACT FROM AUTHOR]