  A compression algorithm is introduced for multi-determinant wave functions
which can greatly reduce the number of determinants that need to be evaluated
in quantum Monte Carlo calculations. We have devised an algorithm with three
levels of compression, the least costly of which yields excellent results in
polynomial time. We demonstrate the usefulness of the compression algorithm for
evaluating multi-determinant wave functions in quantum Monte Carlo
calculations, whose computational cost is reduced by factors of between
1.885(3) and 25.23(4) for the examples studied. We have found evidence of
sub-linear scaling of quantum Monte Carlo calculations with the number of
determinants when the compression algorithm is used.
