Decides which transform to perform, if SpectrumMethod = fourier.
Options:
sine:
Sine transform: $\int dt \sin(wt) f(t)$. Produces the imaginary part of the polarizability.
cosine:
Cosine transform: $\int dt \cos(wt) f(t)$. Produces the real part of the polarizability.
laplace:
Real exponential transform: $\int dt e^{-wt} f(t)$. Produces the real part of the polarizability at imaginary
frequencies, e.g. for Van der Waals $C_6$ coefficients.
This is the only allowed choice for complex scaling.