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This set of parameters, contained in the namelist block &RT_GROUPS, sets radiation group properties for RAMSES RHD runs. Note that the number of photon groups (NGROUPS) is a compilation parameter, to be set in the Makefile. The default settings are HI, HeI and HeII ionizing photon groups with ionisation cross sections and energies derived from a blackbody spectrum with an effective temperature of 10^5 Kelvin.

For detailed descriptions of the concepts described here, see the following papers:

Variable name, syntax, default value	Fortran type	Description
`groupL0 = 13.60,24.59,54.42`	`real array`	Lower energy boundaries, in eV, of each photon group (see §2 in [1]). Used for calculating SED model emission from stellar particles.
`groupL1 = 24.59,54.42,0.0`	`real array`	Upper energy boundaries, in eV, of each photon group. A value of 0.0 is used to represent infinity.
`group_egy = 18.85,35.079,65.666`	`real array`	Average photon energies (eV) for each group. These can either be set manually or left to RAMSES to derive from SED models (with `SEDprops_update>0`).
`group_csn(1,:) = 3.0d-18,0.0,0.0` `group_csn(2,:) = 5.7d-19,4.5d-18,0.0` `group_csn(3,:) = 7.9d-20,1.2d-18,1.1d-18`	`real matrix`	2d matrix representing average ionisation cross sections (cm^2) between each group (first index) and species (second index). These can either be set manually or left to RAMSES to derive from SED models (with `SEDprops_update>0`).
`group_cse(1,:) = 2.8d-18,0.0,0.0` `group_cse(2,:) = 5.0d-19,4.1d-18,0.0` `group_cse(3,:) = 7.4d-20,1.1d-18,1.0d-18`	`real matrix`	2d matrix representing energy weighted average ionisation cross sections (cm^2) between each group (first index) and species (second index). These can either be set manually or left to RAMSES to derive from SED models (with `SEDprops_update>0`).
`spec2group = 1,2,3`	`integer array`	Determines, for each recombining species (HII, HeII, HeIII) which photon group the recombination photons are injected into. Note that recombination emission must be activated with `rt_otsa=.false.` in `$RT_PARAMS`.
====================	====================	Radiation pressure and IR radiation parameters
`kappaAbs = 0.0`	`real array`	Dust absorption (Planck) opacity factor. The real opacity scales with the local metallicity and if `is_kIR_T=.true.`, the IR opacity also varies with the local gas temperature.
`kappaSc = 0.0`	`real array`	Scattering (Rosseland) opacity factor, only used for the IR photon group (which is the first group). The real opacity scales with the local metallicity and if `is_kIR_T=.true.`, it also varies with the local gas temperature.