I've added the possibility to have nucleation reactions in KROME. I've put the nucleating growth and destruction rate functions in krome_grfuncs.f90 as I didn't want to make a new file (because I'm unsure in what places it then needs to be imported as well) and it is related to dust.

These functions assume homogeneous growth via addition of monomers (generalisation might come in a later stage). The growth rate is quite basic (thermal velocity and geometric cross section). The reversed one is based on detailed balance, so it needs ratio's of equilibrium abundances. These latter are determined from Gibbs free energies of the clusters, which are calculated for the most stable configurations of each cluster. This is often just done under standard conditions (1atm, 25°C). Some literature does calculate the temperature dependence of this, but still ignore the pressure dependence. This latter is comes only in to place when calculating the entropy, which needs the partition functions, and it is only in the translational one that the pressure(/volume/number density) comes in to play. This makes it easy to add this extra parameter contribution to the original temperature dependent Gibbs free energy data/fit.

All nucleation stuff/data has been included for TiO2. This will be extended with Al2O3 shortly. Anyone can add any other e.g. SiO also has enough data.