sweep frequencyin ac analysis
The AC simulation only a way to check the Vout/vin relate to the frequency. it don't depends on the loop close or open. such as some circuit's PSRR simulation, it is only AC simulation. Such as .noise analysis. So run AC simulation, you must know what spec you are checking. Check the loop stability you must break the loop, check phase margin and gain. If you only want to know the close loop's Vout/Vin VS. frequency in one stage system, you must close loop.