Dear MESMER-developers,
I am working in kinetics of a complex dissociation reaction using MESMER software and I have some problems with it to calculate the rate coefficient. As it is a dissociation process, I used the prior distribution instead of Boltzmann distribution to describe the energy distribution of the reactant. Meanwhile, our experiment was performed at low temperature (less than 100K). So we set the temperature parameter of MESMER input file to 100K and energyabovethetophill to 362 (The highest energy of the reactions is 10.15 eV). The output went a terrible result--the reactant is -1.67084e+111 and all products were zero in 1e-11 timestep. We are guessing that this result was caused by the wrong energyabovethetophill parameter setting, could you give us some suggestions on how to improve this result?
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Generally speaking, master equation codes have a difficulty with low temperatures because the underlying eigenvalue spectrum has such a wide range of eigenvalue moduli thus making numerical solution difficult. In your case there is the added difficulty of a large energy span. Even so I think that the value of 362 for energyabovethetophill for 100 K seems large, but I do not know the specifics of your system so it hard to judge. You mention the system is a dissociation, then perhaps the use of a reservoir state will help. Also, if you are not doing so already you should be using quadruple precision.
As I say, it is hard for me to know what to say without a few more details. If you are able to share the input file with me I may be able to help a little more.
Thank-you for using MESMER.
Regards, Struan
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Thank you for your reply. I have attached the inputfile for you.
I need to get the branching ratios of all products in a range of energies at low temperature (~100K). However, when I set this temperature to run MESMER, the output file went terrible results--the reactant got 5.61E103 in 10^(-11)s. At 500K, I obtained seemingly reasonable results, but in the experiment, my reactants would remain at around 10% in 10 ^ (-5) s, while the products were quickly consumed in the calculation. Overall, we believe that calculations at a temperature of 100K (up to 200K) are necessary to accurately simulate the experimental environment.
I wish you could give me some advice on how to solve these problems. Thanks.
"Thank you for your reply. I have attached the input file for you. I need to get the branching ratios of all products in a range of energies at low temperature (~100K). However, when I set this temperature to run MESMER, the output file went terrible results--the reactant got 5.61E103 in 10^(-11)s. At 500K, I obtained seemingly reasonable results, but in the experiment, my reactants would remain at around 10% in 10 ^ (-5) s, while the products were quickly consumed in the calculation. Overall, we believe that calculations at a temperature of 100K (up to 200K) are necessary to accurately simulate the experimental environment. I wish you could give me some advice on how to solve these problems. ."
I have now had a chance to look at your input file. I have made a few changes, mainly reducing the size of the systems so that I could see what was happening at the earlier stages of the reaction sequence and running the system for a number temperatures at a fixed gas density. I, also, corrected the definition of the precision to be used. I attach a .zip file containing the results of my calculations. The results can best be viewed in FireFox as described in the manual. From the results you see that at 500 K the systems reacts to give the products 1-04-01-P78 and 1-06-02-83 with the 1-04-01-P78 product being the major product. At 400 K the rate at which these products appear decreases and this trend continues as we move to 300 K. For 200 K and 100 K there is apparently no reaction. Inspection, of the numbers in the .test file shows that for these conditions the initial distribution relaxes to a Boltzmann distribution very quickly and the ratio of 01-Reactant and 1-03-01-02-intermediate84 is that of the equilibrium constant between them. For these condition, the system will eventually decay but it will take a long time. far longer than appears to be happening in your experiment.
There is a number of things to try:
1. Alter the energy of excitation of the initial distribution.
2. Alter the gas density to reduce the rate of stabilization.
3. Alter the barrier heights.
I note also that some of the molecular frequencies are quite low, you might investigate if the associated modes are better treated as internal rotors.
Dear MESMER-developers,
I am working in kinetics of a complex dissociation reaction using MESMER software and I have some problems with it to calculate the rate coefficient. As it is a dissociation process, I used the prior distribution instead of Boltzmann distribution to describe the energy distribution of the reactant. Meanwhile, our experiment was performed at low temperature (less than 100K). So we set the temperature parameter of MESMER input file to 100K and energyabovethetophill to 362 (The highest energy of the reactions is 10.15 eV). The output went a terrible result--the reactant is -1.67084e+111 and all products were zero in 1e-11 timestep. We are guessing that this result was caused by the wrong energyabovethetophill parameter setting, could you give us some suggestions on how to improve this result?
Hello,
Apologies for my slow response.
Generally speaking, master equation codes have a difficulty with low temperatures because the underlying eigenvalue spectrum has such a wide range of eigenvalue moduli thus making numerical solution difficult. In your case there is the added difficulty of a large energy span. Even so I think that the value of 362 for energyabovethetophill for 100 K seems large, but I do not know the specifics of your system so it hard to judge. You mention the system is a dissociation, then perhaps the use of a reservoir state will help. Also, if you are not doing so already you should be using quadruple precision.
As I say, it is hard for me to know what to say without a few more details. If you are able to share the input file with me I may be able to help a little more.
Thank-you for using MESMER.
Regards, Struan
Dear Struan,
Thank you for your reply. I have attached the inputfile for you.
I need to get the branching ratios of all products in a range of energies at low temperature (~100K). However, when I set this temperature to run MESMER, the output file went terrible results--the reactant got 5.61E103 in 10^(-11)s. At 500K, I obtained seemingly reasonable results, but in the experiment, my reactants would remain at around 10% in 10 ^ (-5) s, while the products were quickly consumed in the calculation. Overall, we believe that calculations at a temperature of 100K (up to 200K) are necessary to accurately simulate the experimental environment.
I wish you could give me some advice on how to solve these problems. Thanks.
Hello,
Sorry for my slow reply.
I have now had a chance to look at your input file. I have made a few changes, mainly reducing the size of the systems so that I could see what was happening at the earlier stages of the reaction sequence and running the system for a number temperatures at a fixed gas density. I, also, corrected the definition of the precision to be used. I attach a .zip file containing the results of my calculations. The results can best be viewed in FireFox as described in the manual. From the results you see that at 500 K the systems reacts to give the products 1-04-01-P78 and 1-06-02-83 with the 1-04-01-P78 product being the major product. At 400 K the rate at which these products appear decreases and this trend continues as we move to 300 K. For 200 K and 100 K there is apparently no reaction. Inspection, of the numbers in the .test file shows that for these conditions the initial distribution relaxes to a Boltzmann distribution very quickly and the ratio of 01-Reactant and 1-03-01-02-intermediate84 is that of the equilibrium constant between them. For these condition, the system will eventually decay but it will take a long time. far longer than appears to be happening in your experiment.
There is a number of things to try:
1. Alter the energy of excitation of the initial distribution.
2. Alter the gas density to reduce the rate of stabilization.
3. Alter the barrier heights.
I note also that some of the molecular frequencies are quite low, you might investigate if the associated modes are better treated as internal rotors.
I hope this helps a little.
Regards, Struan
Thank you Struan for your help. I will have a try.