What I want is that the output would be actual AC. I tried adding a -15 V power but the circuit won't work then, saying "timestep too small". Any help is welcome.
To get AC (no dc voltage) you need (after your output) one voltage follower, one capacitor with a resistor to ground and probably one more voltage follower.
A lousy old 741 opamp does not work when an input is at the same voltage as the negative supply pin #4.
The input voltage must be within the alllowed common-mode voltage range which is at least 3V more positive than pin 4 and at least 3V more negative than pin 7.
The input can be at ground when a negative supply is added for pin 4 since then the input will be within the allowed voltage range.