Confidence: It might be this.
Notes: Its difficult to judge the size in your photo, or other details. I can only suggest that common metallic green groundbeetles seen at this time of year include Harpalus, especially H. affinis, and Poecilus cupreus/versicolor.I think the legs on yours are red (as against dark) which would favour Harpalus. If you still have the beetle, I'd suggest starting from there, but from the photo I don't think a confident identification is possible. Good luck!