# 9.1. Calibrating data groups¶

Data calibration is the process of selecting a point in your data as the edge energy and applying an energy shift to your measured data so that the selected point is at a specified value. Thus, it is intended to put your data onto an absolute energy grid. For metals, that value is usually the tabulated edge energy.

When this tool above starts, the current group is plotted as the derivative of μ(E). The menu allows you plot the data as μ(E), normalized μ(E), derivative of μ(E), or second derivative of μ(E). If your data are noisy, you may find it helpful to apply smoothing. IFEFFIT's simple three-point smoothing algorithm is applied the number of times indicated, then the data are replotted.

The selected point is shown in the plot with the orange circle, as
shown in the plot below. You can type in a new value in the
Reference box or click the `Select a point`
button then double-click on a point in the
plot. By default, the Calibrate to box contains the
tabulated edge energy of the absorber measured for these data, but
that too can be edited.

When you plot the second derivative, the `Find zero
crossing` button becomes enabled. This finds the zero crossing
of the second derivative that is nearest to the current value of the
reference point. When plotting the second derivative, smoothing helps,
even for fairly clean data.

When you click the `Calibrate` button, the values of
`«E0»` and `«Energy shift»` are set for the current
group such that the selected point in the data takes the calibration
value.

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