  We develop an extension of eigenvector continuation (EC) that makes it
possible to extrapolate simulations of quantum systems in finite periodic boxes
across large ranges of box sizes. The formal justification for this approach,
which we call finite-volume eigenvector continuation (FVEC), is provided by
matching periodic functions at different box sizes. As concrete FVEC
implementation we use a discrete variable representation based on plane-wave
states and present several applications calculated within this framework.
