  Eigenvector continuation EC has been shown to accurately and efficiently
reproduce ground states for targeted sets of Hamiltonian parameters. It uses as
variational basis vectors the corresponding ground-state eigensolutions from
selected other sets of parameters. Here we extend the EC approach to scattering
using the Kohn variational principle. We first test it using a model for S-wave
nucleon-nucleon scattering and then demonstrate that it also works to give
accurate predictions for non-local potentials, charged-particle scattering,
complex optical potentials, and higher partial waves. These proofs-of-principle
validate EC as an accurate emulator for applying Bayesian inference to
parameter estimation constrained by scattering observables. The efficiency of
such emulators is because the accuracy is achieved with a small number of
variational basis elements and the central computations are just linear algebra
calculations in the space spanned by this basis.
