Update README.txt

sample/input/descriptor_vectors/H-W/README.txt

@@ -10,4 +10,19 @@ obtained as follows:
             a) bcc W atom
             b) H atom in a tetrahedral instersitial site
             c) W atom as first nearest neighbor to the H atom
-            d) W atom as second nearest neighbor to the H atom
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+            d) W atom as second nearest neighbor to the H atom
+            
+Attention need to be paid to the quip command line to compute the 
+DVs for the hydrogenated W sample
+The command line is:
+     
+     quip atoms_filename=interstitial_geometry.xyz descriptor_str="soap cutoff=3.0 l_max=4 n_max=4 atom_sigma=0.5 n_Z=2 Z={74 1} n_species=2 species_Z={74 1}"
+
+where:
+    n_Z=2 is defined for 2 species in the material sample, this value can be changed
+          for material with more than 2 species.
+    Z={74 1} this parameter is defined according to the charge of the species, here 
+             we consider that W = 74 and H = 1. The order of these number needs to be
+             defined according to the list of atoms defined in the xyz file. 
+    n_species=2 this is the number of species in the material
+    species_Z={74 1}  the order for this parameter is defined by the xyz file.
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