Feedback factor of Op-Amp based Colpitts Oscillator
niki - of course, your test circuit cannot work because you have connected an ideal ac source to the low-resistive output of the opamp (shorting the opamp output). For finding the loop gain the feedback loop must be opened at a suitable node for injecting the test signal at this node.

The problem in your circuit is that we have two feedback ways (forget the term "forward" in your figure - it`s nonsense). Hence, opening both feedback loops (which is, theoretically necessary) will cause problems with the very large open-loop gain Aol of the opamp. Hence, we need a method which would work also for the idealized case with Aol approaching infinite.

Therefore, we must treat the opamp with feedback as a finit gain amplifier with an unknown gain which is frequency-dependent and determined by ALL elements.

Method: Open the loop at the left (correction: right) node of R3 and inject the test signal Vin. The ratio Vout/Vin gives you the loop gain which must have a magnitude of slightly above unity at the zero-phase frequency. Tune this gain with R2.