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for opamp, i did not see anything wrong to do open loop when you sim ac (i mean for simulation purpose only). for transient, you'd better do close loop especially when you deal with high gain opamp.
when the opamp has very high gains(around 80 dB or more) opening the loop can saturate opamp. you must change the gate of one transistors of input stage very carefully(around microvolts)
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.
AC analysis is the frequency sweep. It is used to get frequency response. It depends on what kind of frequency response, then the system can be closed loop or open loop or even off for switch ( such as rejection or cross coupling,etc).
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