1.what is the difference between these two restraints in terms (when to
use) usage ?
Soft sphere penalizes interpenetration of two atoms's vdw spheres using
a simple harmonic function. Lennard Jones uses the full 6-12 potential.
Soft sphere is cheaper since you can use a shorter nonbond cutoff
distance, but less accurate because you miss the long range attractive
component. You can use whichever one you want - it doesn't make a huge
amount of difference.