Function parallel_verlet_step 

pub fn parallel_verlet_step(
    positions: &mut Vec<[f64; 3]>,
    velocities: &mut Vec<[f64; 3]>,
    forces: &[[f64; 3]],
    masses: &[f64],
    dt: f64,
)

Parallel velocity-Verlet position and velocity half-update.

Updates positions with x += v*dt + 0.5*a*dt^2 and velocities with v += 0.5*a*dt (first half of the Verlet velocity update; call again after recomputing forces for the second half).

The loop runs in parallel via par_iter_mut.