I was simulating a ZVS driver in LTspice and while the frequency of the output waveform matched up with f = 1 / (2π √LC) when the transformer had not been added yet. With L and C being the value of the inductor and capacitor at the output respectively. Taking C as 1 µF and L as 4 µH, f is equal to 79.577 kHz.
After adding the transformer, the frequency changed. By taking the time between the peaks of the waveform to be 17.592 µs, and using the formula 1/T, you get 56.844 kHz as the frequency.
Hence my question: What exactly causes this and how can I calculate the frequency properly?