Number density of collision partners (usually H_{2}). By
default, collisions with He are not taken into account. To approximate
their effect, multiply the H_{2} density by 1.2 for an
He/H_{2}=0.2 abundance. The effects of mass and geometrical
cross section between He and H_{2} cancel out to first order.

Unit: cm^{-3}.

Allowed range: 10^{-3} - 10^{13}