The STAND2020 network

The STAND2020 network (Rimmer & Helling 2016; Hobbs et al. 2021; Rimmer et al. 2021) has additional types of reactions, in particular the following 3-body reactions:

  • B = Neutral Termolecular and Thermal Decomposition Reactions
  • BS = Neutral Termolecular and Thermal Decomposition Reactions (added to model Venus)
  • BI = Neutral Termolecular and Thermal Decomposition Reactions (added to model Isoprene)
  • G = Reverse Neutral Termolecular and Thermal Decomposition Reactions
  • Q = Ion-Neutral Termolecular and Thermal Decomposition Reactions
  • R = Reverse Ion-Neutral Termolecular and Thermal Decomposition Reactions
  • T = Three-Body Recombination Reactions
  • I = Thermal Ionization Reactions

These reaction rates come with low-p and high-p Arrhenius coefficients, i.e. 2x3 parameter for each reaction. Both forward and reverse reaction directions are included, i.e. two reactions for each reaction pair. The reverse rates (type G, R, I) are just placeholders and come with copied (invalid) reaction data. To use these reactions in ProDiMo, say in Parameter.in:

.true.     ! STAND     : use STAND2020 network 
.true.     ! STANDover : overwrite 
B BS BI G Q R T I     ! STANDfilter

When you activate STAND, the following actions are undertaken:

  1. read the BURCAT thermochemical data for Gibbs(T) from STAND2020_Thermo.dat
  2. use the dictionary STAND2020_Dictionary.dat to translate species names
  3. read the reactions and Arrhenius parameters from STAND2020_Reactions.dat
  4. automatically add all other reverse gasphase reactions
  5. identify forward-reverse pairs of all gasphase reactions

The standard output/log file should contain lines like this

having added       166 reactions
having replaced     18 reactions
having created    1490 reverse reactions
having identified 1666 reaction pairs
altogether        3960 chemical reactions
number of unpaired gasphase reactions 286

With further details in ChemInit.log. After initialisation, the following new reaction types are used in ProDiMo

BF:  forward from STAND B,BS,BI           
BR:  reverse of BF, from STAND G          
QF:  forward from STAND Q                 
QR:  reverse of QF, from STAND R          
AF:  forward form STAND T                 
AR:  reverse of AF, from STAND I          
AA:  reverse of ProDiMo gasphase reaction

For each pair of reactions, ProDiMo will calculate (a) the low-p and high-p reaction rates and (b) dG = the change of Gibbs free energy for one reaction. Then, a decision will be made, based on dG and availability of valid reaction data, which reaction is "forward" (exotherm) and "reverse" (endotherm). The reverse rates are then computed from the forward rates times a factor that is given by the dG. This procedure makes sure that the network will approach thermo-chemical equilibrium for very high densities.

We have carefully tested that this is the case by using the independent thermo-chemical equilibrium code GGchem (Woitke+ 2018) is the case for 700K and 2000K. Here is a logfile of such a test

n = 3.4E+14 cm-3 
T = 700 K using 
Jv=Bv(T) in ProDiMo 
using tiny CRI in ProDiMo 
using tiny LX in ProDiMo 
using tiny fPAH in ProDiMo 
using abund_ProDiMo.in in GGchem 
using BURCAT data in GGchem 
removing H2exc, ices, H+H+dust

      GGchem GGchem(BURCAT) ProDiMo
H    1.069E+03 1.070E+03 1.070E+03 
H2   1.732E+14 1.732E+14 1.732E+14 
O    3.187E-16 3.197E-16 3.198E-16 
OH   2.997E-05 4.016E-05 4.016E-05 
H2O  5.682E+10 5.682E+10 5.683E+10 
O2   7.664E-20 7.692E-20 7.688E-20 
He   3.341E+13 3.341E+13 3.341E+13 
C    2.521E-29 2.528E-29 2.526E-29 
CO   4.754E+10 4.754E+10 4.755E+10 
CO2  1.476E+08 1.468E+08 1.468E+08 
CH4  1.511E+08 1.480E+08 1.481E+08 
CH   9.064E-25 6.824E-25 6.375E-25 
C2H2 8.106E-05 6.228E-05 6.241E-05 
N    2.491E-18 2.501E-18 2.500E-18 
N2   1.376E+10 1.376E+10 1.377E+10 
NH   1.374E-15 2.913E-14 2.914E-14 
NH2  6.301E-07 1.322E-06 1.323E-06 
NH3  2.404E+05 2.422E+05 2.422E+05 
HCN  4.780E+01 1.195E+02 1.197E+02 
CN   1.615E-15 8.788E-16 8.801E-16 
Ne   3.089E+10 3.089E+10 3.089E+10 
Na   7.938E+05 7.926E+05 7.940E+05 
Ar   4.167E+08 4.167E+08 4.167E+08 
Mg   2.981E+06 3.653E+06 3.714E+06 
S    2.985E-05 2.905E-05 2.905E-05 
HS   2.007E+01 1.303E+01 1.303E+01 
H2S  6.454E+07 6.454E+07 6.454E+07 
OCS  6.211E+02 1.069E+03 1.070E+03 
CS   7.524E-06 9.904E-06 9.911E-06 
e-   1.886E-06 1.882E-06 1.886E-06 `

The chemical analysis shows that, as expected, we find detailed balance

input species to analyse ... CH 
gas temperature = 7.000E+02 
H nuclei density = 3.466E+14 
chi = 1.000E-99 
abundant species: H2 He CO N2 H2O H2S Mg Fe Na Ne Ar 
                                      rate coeff rate[cm-3s-1] 
formation of CH ... 
1747 5387  NN: H + CH2 --> CH + H2      2.20E-10  1.45E-21 
3960 31490 AA: H2CO + H --> CH + H2O    9.62E-28  6.52E-25 
2356 20315 BR: CH3 + M --> CH + H2 + M  5.19E-38  1.29E-25 
3784 31314 AA: C2H4 + H --> CH + CH4    1.06E-27  8.79E-27 
3799 31329 AA: CO + CS + H --> CH + OCS 5.41E-40  2.73E-31 
1731 5365  NN: H2 + C --> CH + H        1.82E-17  7.96E-32 
destruction of CH ... 
1734 5368  NN: H2 + CH --> CH2 + H      1.31E-11 -1.45E-21 
2304 14000 NN: CH + H2O --> H2CO + H    1.80E-11 -6.52E-25 
2095 20028 BF: CH + H2 + M --> CH3 + M  5.63E-30 -1.29E-25 
1693 5305  NN: CH + CH4 --> C2H4 + H    9.31E-11 -8.79E-27 
2059 6123  RA: H2 + CH --> CH3 + PHOTON 4.90E-18 -5.41E-28 
1710 5329  NN: CH + OCS --> CO + CS + H 4.00E-10 -2.73E-31 
particle density = 6.375E-25 concentration = 1.839E-39 
total rate = 3.150E-38 3.150E-38 cm^-3/s 
total formation,destruction = 1.453E-21 1.453E-21 cm^-3/s