New cat chasing resident cat

Question

I have adopted my daughter’s cat Velma, she’s eight months old, I had her spayed a couple weeks ago. Resident cat Theo is very gentle and mellow. Velma has been here for seven weeks, I did the slow introduction thing, and I allowed them together twice. Both times Velma chased Theo, hissing, swatting and trapping Theo under the bed. Velma is mean when she’s doing this, and she slinks away afterwards. I realize Velma is scared and unsocialized. I will not allow Velma to do this to Theo again. Now, I keep them separated, but it’s exhausting. Any advice??