Hi Saeed, I’ll copy the LAMMPS mailing list here, in case somebody can help you better.
Can you specify the functional form of the order parameter you would want to use for “density”? There is a recent addition here: https://colvars.github.io/colvars-refman-namd/colvars-refman-namd.html#sec:cvc_volmaps
but it is unfortunately specific to NAMD so far.
The bond order parameter has been discussed already in the following thread, which is admittedly not easy to find because the original poster used a very generic title: https://lammps.sandia.gov/threads/msg83669.html
The short answer is, there is at present no implementation of Q6 for Colvars, but you could try using the implementation of Q6 from Plumed: it is still labeled “experimental” but perhaps will be more mature soone. If there are any issues with it, consider posting your feedback to the Plumed mailing list.