[gmx-users] question about g_rdf

[lammps lammps]

Dear everyone,

I use the command (g_rdf -f traj.trr -s md.tpr -o rdf_12.xvg -sq sq_12.xvg
-b  1000 -bin 0.01 -cut 1.2 -grid 0.05 ) to calculate the RDF

There are two questions:

Question_1: Why should we do the three times selections[ the red line],

Reading file md.tpr, VERSION 4.0.4 (single precision)
Reading file md.tpr, VERSION 4.0.4 (single precision)

Select 1 group【the first time, what aim is this selection? 】
Opening library file /home/tianwd/soft/gromacs
4//share/gromacs/top/aminoacids.dat
Group     0 (      System) has  5240 elements
Group     1 (         SOL) has  5216 elements
Group     2 (         CL-) has    12 elements
Group     3 (         NA+) has    12 elements
Select a group: 3
Selected 3: 'NA+'
trn version: GMX_trn_file (single precision)

Reading frame    1500 time 3000.000
Last frame       1500 time 3000.000
Reading file md.tpr, VERSION 4.0.4 (single precision)

Select a reference group and 1 group【the second time must select two groups
】
Opening library file
/home/tianwd/soft/gromacs4//share/gromacs/top/aminoacids.dat
Group     0 (      System) has  5240 elements
Group     1 (         SOL) has  5216 elements
Group     2 (         CL-) has    12 elements
Group     3 (         NA+) has    12 elements
Select a group: 1
Selected 1: 'SOL'
Select a group: 3
Selected 3: 'NA+'
Reading frame     600 time 1200.000


Q_2: What is the reasons for  the following error?


Select 1 group
Opening library file
/home/tianwd/soft/gromacs4//share/gromacs/top/aminoacids.dat
Group     0 (      System) has  5204 elements
Group     1 (         SOL) has  5168 elements
Group     2 (         CL-) has    18 elements
Group     3 (         NA+) has    18 elements
Select a group: 0
Selected 0: 'System'
Reading frame       0 time 1000.000
-------------------------------------------------------
Program g_rdf, VERSION 4.0.4
Source code file: gmx_rdf.c, line: 639

Fatal error:

Error: atom (MW) not in list (5 types checked)!

-------------------------------------------------------


Thanks in advance
-- 
wende
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