Failing Geometry optimization with fixed atoms using the GFN-FF

[benbaed]

Describe the bug
Geometry optimizations using the GFN-FF do not work when I try to fix certain atoms of the structure.
A possible work-around is to use the ANC optimizer and not the default L-ANC optimizer.

To Reproduce
Steps to reproduce the behavior:

This consistently happens with multiple of my NCI structures (I could not reproduce it using Benzene)

1. start xtb (version 6.5.1) with:
   xtb struc.xyz --gfnff --gbsa toluene --opt vtight --input xtb.inp > xtb.out

2. the following errors are printed:
   [ERROR] Program stopped due to fatal error
   -3- Geometry optimization failed
   -2- xtb_geoopt: Geometry optimization did not converge
   -1- gfnff_setup: Could not read topology file.

3. Searching for "L-ANC optimizer", one can find the following message:
   k= 528 nvar= 4974
   ANC generation failed

Expected behaviour
As using the normal ANC optimizer, the given atoms should be fixed and the optimization should be running and not stop.

Additional context
I found that using the normal ANC optimizer solves this issue, the xtb.inp then looks like this:

$opt
 engine=rf
$fix
  atoms: 1-1482
$end 

Since the ANC optimizer is the default for gfn2, I did not observe the bug there.

Input and generated files
gfnff_fix_bug.zip

[marcelmbn]

@benbaed @Thomas3R Is this issue still present?