Number density of collision partners (usually H2). By
default, collisions with He are not taken into account. To approximate
their effect, multiply the H2 density by 1.2 for an
He/H2=0.2 abundance. The effects of mass and geometrical
cross section between He and H2 cancel out to first order.
Unit: cm-3.
Allowed range: 10-3 - 1013