  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 ans\"{a}tze and modifying them
to construct novel wavefunction forms. FANCI provides a powerful framework for
exploring, developing, and testing new wavefunction forms.
