Editing RDF (section)

== x.RDF: potential, quirks and limitations ==

The program can:
 1) compute a maximum of 10 (max_Cat) RDF with a grid of 10000 (max_Seg) points on primitive cells with at most 40 (max_Elem) groups, none of which can have more than 1000 (max_At) 
    atoms. If the atoms are connected (conn=T/t/Y/y), the maximum ratio is 1:8. If asked for a BI-dimensional RDF, the program can find at most 10 layers (max_L). If you find some 
    of this values too small, please go to the file ''rdf.i'' inside the directory RDF and change the corresponding value. For example, if you think that your system will have more
    or less 15 layers, put "max_L = 20"" in rdf.i, and COMPILE THE PROGRAM AGAIN.

 2) compute the RDF of a group of atoms with itself. In line 13 you just have to copy the first 5 columns in the columns 6-10. For example, in Figure 1, line 14 and 15 are two perfect
    examples for the computation of RDF of one group with itself (line 14: RDF(OO); line 15: RDF(HH)).

 3) Whereas TRI-dimensional RDF (3D-RDF) is directionless, the BI-dimensional one (2D-RDF) is not, and assumes that the layers are ordered along the Z axis. If this is not the case, and they are
    ordered along another axis, please rotate the system so that the new Z axis coincide with the ordering direction.

 4) In the case of 2D-RDF, if the groups are not connected, the layers are separated by nodal planes along the Z axis. However, if the groups are connected, the layers are separated by wavy surfaces,
    so that the first group of atoms is divided according to the Z and dZ value (see line 2 in ''param.dat''), and the second group of atoms is added to the layer on which their connected atom belongs.
    For example, suppose that the surfaces that divide the layers are at 6.0, 7.5 and 9.0 angstroms, and in the first group we have 3 oxygens (o1, o2, o3) with Z: 5.5, 6.5, and 7.8. o1 will go in
    the first layer (Z(o1) is below 6 Ang), o2 in the second (its Z is between 6.0 and 7.5), and o3 in the third (7.5 < Z(o3) < 9.0).
    Each of these atoms is connected to 2 H: h11 (Z=5.0), h12 (Z=5.2), h21 (Z=5.9), h22 (Z=6.9), h31 (Z=8.1), h32 (Z=7.2).
    Because you specified that the atoms are connected, he will put h11 and h12 in the first layer with their oxygen, h21 and h22 in the second layer, and h31 and h32 in the third layer, even if 
    h21 should be in the first layer (Z(h21) < 6.0) and h32 in the second layer (6.0 < Z(h32) < 7.5).
    If you specified that the atoms are not connected (conn=N/n/F/f), then the outcome is different: the program will place o1, h11, h12 and h21 in the first layer (Z < 6.0); o2, h21 and h32 in
    the second layer (6.0 < Z < 7.5); and finally, o3 and h31 in the third layer (Z > 7.5).

 5) ''Quirk'': the program order the first and the second group so that the ratio is 1:N. So, if you put as first group 64 O (of water) and 128 H (of water), the ratio is 1:2, and it's ok for the program. Nevertheless, if
    you reverse the order (1st group: 128 H; 2nd group: 64 O), the ratio is 2:1, and the program WILL SWAP THE GROUPS... Don't worry, it will tell you, but remember it. 

 6) Limitation: the program computes RDF for connected systems of ratio 1:N; if the ratio is different, for example 2:5, the program will stop and do not compute anything. :(