Dear Harshita,

There could be several reasons why oxygen molecules are dissociating before getting adsorbed on the palladium surface in your LAMMPS simulation. Some possible issues are: Insufficient time: It is possible that you are not allowing the system enough time to equilibrate at each temperature. You may need to run the simulation for a longer time or increase the timestep to ensure that the system reaches equilibrium. Incorrect force field: The force field you are using may not accurately capture the interactions between the oxygen molecule and the palladium surface. You may want to check if there are any parameters missing or if you need to modify the force field parameters to better represent the system. Inappropriate thermostat: The Berendsen thermostat may not be appropriate for your simulation. You could try using other thermostats like the Nosé-Hoover thermostat or the Langevin thermostat to see if it makes a difference. Inappropriate initial conditions: It could be that the initial conditions of your simulation are not appropriate for the system you are studying. You may want to try different initial configurations to see if the dissociation of oxygen molecules still occurs.

This usually happens because of very high forces that will cause your molecule to dissociate. i notice you not using qeq probably because of deposition but part of the oxygen molecule binding is electrostatic. Disperse some oxygen molecules and test at that temperature which you observe dissociation but with qeq and see if the same behavior happens.

another cause might me that the reaxff potential is not parametrized for these high temperatures.

maybe your molecule settings for oxygen is not consistent with the prediction of the potential.

if the problem happens to be because of qeq you can discard your deposition option and add molecules at the top of the box then use the add force command to push them to the surface.

there are many possibilities, you need to narrow down the problem. I just gave you some possibilities.