Hello Prof Gale,
I have a 2d Ag slab with Co2 on it. I am using the Co2 shell model, however, I am not fitting the same here. I am fitting Ag/C where C is carbon core. I am testing with 3 configs but getting an error when I run it. I do not understand why. I am attaching the file.
Ag_CO2_1.txt (14.5 KB)
I have resolved the issue but I made 2 observations in running a core-shell model.
- I am not fitting the core-shell. I am fitting Ag with carbon cores. I took coords of shell and core of co2 from lammps. However, when I tried to run a simple fit of Ag/C with fbfgs, I got errors. Only when I set the coods of both coord and shell same, did it run. However, on observing the examples I noticed, that say eg 2 of GULP, that c/s coords are not same but eg1 of GULP has same coords and they both run. So why did I get an error?
- I observed that in some example files, charge neutrality for c/s model holds (eg 4) and others it doesn’t (eg 2). Yet the files run in both cases. But when I made a mistake of not maintaing charge neutrality in my system, the run stops with an error. Why is this?
Thanks a lot,
There are a lot of points here, but importantly I’d encourage you to read the Phil. Mag. B paper about GULP fitting (and fitting in general) since this covers many issues. Looking at your original input there are several concerns:
- You are fitting energies of 3 configurations that all have energies of zero! Note that 10^-23 kcal is zero as values below about 3 decimal places would be small compared with thermal energy and well below the accuracy of most QM methods, etc.
- By using weights of 10^-7 it means your contribution to the sum of squares would be negligible and so not useful.
- By using standard fitting (see simultaneous and relax fitting in the paper/manual) it means you are basically just fitting the forces to be zero at the structures you have given, but if they are 3 structures for the same system then this doesn’t make sense.
- I wouldn’t use “fbfgs” since this will take longer and you don’t necessarily need off diagonal numerical Hessian matrix elements.
The bottom line is that you don’t have enough data for your fit and it generally doesn’t make sense.
Now to charge neutrality - you need to read up on the Ewald (or in this case Parry for 2-D) sum and this will explain why things not being charge neutral is a problem. Think about the fact that you are working in periodic boundary conditions and so you have an infinite system and what would happen if you have an infinite charge (think along the lines of explosion!).