Function density_accumulation 

pub fn density_accumulation(
    positions: &[[f64; 3]],
    masses: &[f64],
    h: f64,
    neighbor_list: &NeighborList,
) -> Vec<f64>

Accumulate density contributions from a neighbour list.

For each particle i, sums contributions from its neighbours j: rho_i += m_j * W(|r_i - r_j|, h)

Only pairs within distance h contribute. The neighbour list is expected to already be built for the given positions.