Problem with a Relaxation Oscillator (using Op Amp)
Your formula is written such that it assumes that Vdd and Vss are symmetric with respect to ground. If this is not true &mdash; e.g., you are using only a single power supply with Vss tied to ground &mdash; this formula does not apply.

If you want to use this circuit with a single supply (or asymmetric supplies), you'll need to connect the grounded end of R1 to a "virtual ground" at (Vdd+Vss)/2. In fact, what you can do is simply split R1 into two separate resistors with twice the value, and connect one between R2 and Vss and the other between R2 and Vdd.