Hi Prof Gale,
I have gone through the manual and the examples provided and I must say it’s quite a multifaceted software. I am actually inspired by the amount of thought and discussions and foresight that must have gone behind this creation.
Now to the problem  I want to fit the parameters of a rigid framework and the coreshell model of a molecule to the DFT energies of each config. Regarding the same, I have 3 queries 

The GULP manual talks of using ‘simul fit’ and taking the shell position as fitting parameters. However, can I do the following  I generate the shell positions beforehand through LAMMPS and input them as ‘atomtype’ ‘shell’ ‘x y z’ and use fit instead of simul fit?

I went through Example 4 and just out of curiosity, therein simply changed simul fit to fit. However, in the output, I got the error “Cell parameter has fallen below allowed limit”. Can you please enlighten me as to why this happened?

I went through the output of Example 4 and found two very ‘curious’ mentions (I could not make sense of them)
I)
Observable no. Type Observable Weight Reference Config
1 Derivative 0.000000 1.0000 1 z 1
2 Derivative 0.000000 1.0000 3 z 1
3 Derivative 0.000000 1.0000 4 z 1
4 Derivative 0.000000 1.0000 6 z 1
5 Derivative 0.000000 1.0000 a 1
6 Derivative 0.000000 1.0000 c 1
I could not understand the mentions under the label ‘Reference’ . a, c , 'n’z ? The nature of the observables I mean.
Later on in the table of variables, I saw that 4 z and 6 z are shell positions. However, I do not understand why only the z coords of the shells are considered as parameters and optimized.
II) I also saw a table of ‘Final values of residuals’. I searched the help manual but could not understand the meaning of residuals here.
I am looking forward to your answers and ‘GULPing’ onwards. Thanks in advance.
Deepest regards,
Nandana