My partner and I recently got a female cat of less than one year. We gave her a few days to explore the house, trading her and my female of 1.5 years out in the different parts of the house. Then we did the baby gate thing, whereupon they hissed and eventually just settled down to look at each other.

When the baby gate was removed, there was a lot of hissing and more quiet looking, but eventually our bigger cat began to chase her and we've not been able to get her to stop since. We just separated them after a few of these loud and nasty chases, and we're not sure how to progress from here. How do we make the chases cease?

Pic related--my cat on the right, our new baby on the left.