$FIX: not respecting fixation in xcontrol file anymore (v.6.7.1)

[JayTheDog]

Describe the bug When trying to optimize only H-Atoms, xtb does not respect the fixation of atoms in place anymore. I tested also 6.7.0, where this issue is not occurring.

To Reproduce xtb glycineAcid.xyz --input glycineAcid.xcontrol --opt

The xyz file:

10

C              0.0834925009735182             0.0000000000000000            -1.6668515099899917
O              0.0834925009735182            -1.1445070789971263            -2.0630581700150108
O              0.0834925009735182             1.0691091199790088            -2.4983246289908534
C              0.0834925009735182             0.4230545059847334            -0.2020795040210518
H              0.9565438779960600             1.0664427810136214            -0.0395754850004047
N              0.0834925009735182            -0.6677380050243458             0.7545830369902874
H              0.8900621800066862            -1.2628578009803400             0.5783254700023934
H             -0.7230771790121688            -1.2628578009803400             0.5783254700023934
H             -0.7895588770015425             1.0664427810136214            -0.0395754850004047
H              0.0834928001703133            -0.2969331876126342             1.6184980029345206

The xcontrol file:

$samerand
$fix
  elements: C, N, O
$end

Expected behaviour No movement in the atoms except for H-Atoms.

Output shows:

 Lowest  eigenvalues of input Hessian
    0.000000    0.000000    0.000000    0.000000    0.000000    0.000000
    0.012712    0.016253    0.017576    0.047909    0.064565    0.074817
    0.090051    0.112446    0.112874    0.127281    0.155430    0.226076

Expected (and in older versions):

Lowest  eigenvalues of input Hessian
  -0.000000   -0.000000   -0.000000   -0.000000   -0.000000   -0.000000
   0.000000    0.000000    0.000000    0.000000    0.000000    0.000000
   0.000000    0.000000    0.000000    0.000000    0.000000    0.000000

[cplett]

This bug seems to be related to the default Approximate Normal Coordinate Rational Function optimizer (ANCopt) in xtb version 6.7.1. A way to still do the exact fixing with this xtb version is to use another optimization engine like L-ANCopt or FIRE.