initial_guess_hwhm

Syntax

initial_guess_hwhm = 0.001
Meaning

Initial half-width at half maximum (HWHM) used when pySED starts a Lorentzian fit. If fits fail or converge to unreasonable widths, this value can help guide the optimizer.

Default

0.001.

Notes

This value is the starting guess for the Lorentzian half-width at half-maximum, passed to scipy.optimize.curve_fit. It has units of THz in the current pySED fitting workflow.

In the SED theory used by pySED, the fitted peak is written as a Lorentzian

\[\Phi(\mathbf{q},\omega) = \frac{I} {1+\left[(\omega-\omega_c)/\gamma\right]^2},\]

where \(\gamma\) is the HWHM. The PYSED paper defines the lifetime from this linewidth as \(\tau = 1/(2\gamma)\). In pySED output, frequencies are handled in THz and lifetimes are written in ps using the code convention described in the theory page.

A reasonable initial HWHM helps the nonlinear fit converge to the physical linewidth. If initial_guess_hwhm is much too small, the fit may lock onto a very narrow spike or fail for broadened peaks. If it is much too large, the fit can over-broaden nearby peaks or converge slowly. For sharp crystalline peaks, values such as 0.0005 to 0.005 THz are often a useful starting range, but the best value depends on the material, temperature, trajectory length, and frequency resolution.

Tune this parameter only after peak_height and peak_prominence detect the correct peaks. Then check the fitted curve visually before using lorentz_fit_all_qpoint = 1.

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