diff --git a/ChangeLog b/ChangeLog
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+2024-03-12 Manfred Brath <manfred.brath@uni-hamburg.de>
+
+    * ARTS-XML-DATA-2.6-3
+	
+	* planets/Earth/Optimized-Flux-Frequencies:
+	
+	Add readme.
+	
 2024-03-12 Manfred Brath <manfred.brath@uni-hamburg.de>
 
 	* ARTS-XML-DATA-2.6-2
diff --git a/planets/Earth/Optimized-Flux-Frequencies/readme b/planets/Earth/Optimized-Flux-Frequencies/readme
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+This folder contains optimized set of frequencies and quadrature weights
+from Paulina Czarnecki for fast long and shortwave flux calculations.
+
+Use the frequencies as f_grid for the radiative transfer simulation and 
+the weights for the spectral integration.
+
+Originally the frequencies and weights were in the units of wavenumbers but 
+they are converted to Hz.
+
+For details on the algorithm for the frequency selection and weight estimation
+see:
+P. Czarnecki, L. Polvani, R. Pincus, Sparse, empirically optimized quadrature 
+for broadband spectral integration, Journal of Advances in Modeling Earth 
+Systems 15 (10) (2023) https://doi.org/10.1029/2023MS003819