There are now no impurities in the crystal but "born" can add a little anharmonicity to ALL of the springs in the chain. If the quartic constant is zero, the chain is purely harmonic. If the constant is one, then the forces due to the quartic term in the potential are comparable with those due to the quadratic term for unit displacement amplitude.
The preset leaves the quartic potential equal to zero to remind you, when you run, that too narrow a packet quickly disperses. Now set the quartic constant equal to 4 and run again. Note that the chain is perfectly uniform: ALL of the springs are anharmonic! Try choosing a different peak position for the initial packet. Why does the packet always stay where it's started?