Determining Power Spectral Density from Autocorrelation Function and Time-Series Data
Taking the FFT of a time-series does not give you the power spectral density (PSD). What you can do is take the squared magnitude of the FFT to get an estimate of the PSD. And the latter is equivalent to taking the Fourier transform of the (deterministic) autocorrelation function of the data. See also this answer to a related question.

So the inverse Fourier transform of the PSD is the autocorrelation function.