After some answers from other users and by playing around with the initial configuration of the system, I eventually got this to work. Here are input files for the simulation of an equimolar mixture of CO2 and CH4 (TraPPE force field) for your reference. The initial configuration was generated using Packmol.
I am including Lammps list in the loop, since others might also be interested in this.
lammps.data (54.3 KB)
lammps.in (482 Bytes)
log.lammps (38.6 KB)