In cometary comas and in some protoplanetary and debris disks, collisional excitation can be dominated by H_{2}O and/or CO molecules.
Just like H_{2}, water molecules exist in two isomeric forms: para-water with total nuclear spin *I* = 0 and ortho-water with *I* = 1.
The rotational energy levels of H_{2}O are labeled by three numbers: the angular momentum *j* and the pseudo-quantum numbers *k _{a}* and

*k*(corresponding to the projection of

_{c}*j*along the principal inertia

*a*and

*c*axes). In the ground electronic and vibrational state

^{1}, the rotational levels of para-H

_{2}O have even values of

*k*+

_{a}*k*while the levels of ortho-H

_{c}_{2}O have odd values of

*k*+

_{a}*k*. The rotational energy levels of CO are labelled by the angular momentum

_{c}*j*.

In contrast to H_{2}, even in cold environments where the kinetic temperature *T _{k}* is lower than 50 K,
several rotational states of H

_{2}O and CO can be significantly populated due to the relatively small rotational spacings in these species (compared to those of para- and ortho-H

_{2}). As a result, several excited levels of H

_{2}O and CO must be considered as collision partners in radiative transfer calculations.

**EMAA provides de-excitation rate coefficients (in cm ^{3}s^{-1}) due to collisions with 'para-H_{2}O' and 'ortho-H_{2}O' and
'CO' which in practice correspond (for each species) to 'thermalized' rate coefficients, >i.e. summed over all possible final states and averaged over a thermal
distribution (at the kinetic temperature) of initial states (see Faure et al. 2020 and Żółtowski and al. 2023).**

Note: H_{2}O and CO are not handled as a colliding partner in the public version of the RADEX^{2} code.
In order to use RADEX with para-H_{2}O and or ortho-H_{2}O as projectiles, please select 'p-H_{2}' and/or 'o-H_{2}' as collision partners in the RADEX input file (corresponding to id=2 and id=3 respectively, where 'id' is the code of collison partners).
This is obviously only possible if H_{2} and H_{2}O are simultaneous collision partners.
In order to use RADEX with CO as projectile, please select 'H' as collision partners (id=5) in the input file.
Again, this is only possible if H and CO are not simultaneous collision partners.

### References

Faure A., Lique F., Loreau J., *MNRAS* **493** 776-782 (2020)

Żółtowski M., Lique F., Loreau J, Faure A., Cordiner M.,* MNRAS* **520** 3887-3894 (2023)

^{1}The first vibrational level of H_{2}O opens at 1594.7 cm^{-1}

^{2}https://home.strw.leidenuniv.nl/~moldata/radex.html