AIMD for reaction

Questions
#1

Hello, I would like to determine the time for a reaction (reactant through TS to product) using AIMD and compare it to experimental results. I started from the TS then to determine the reaction time from TS to reactant and to product. I will run 100 times for each, then the total reaction time should be the sum of the average for each reaction direction. But I have two questions in using QChem for AIMD.

(1) First, I run the frequency calculation at TS. Then, in the BOMD step, I use AIMD_INIT_VELOC=ZPE to include and partition ZPVE among the non-imaginary vibrational modes. In addition, I need to specify the "transition vector energy” through REM_AIMD_KINETIC_ENERGY. How do I determine the value for REM_AIMD_KINETIC_ENERGY?

(2) How do I control the motion along the transition vector in the direction toward the product or reactant?

I also tried to use Quasiclassical MD by setting AIMD_INIT_VELOC=quasiclassical. But Quasiclassical MD is not yet supported for transition states in QChem.

Thanks!

#2

When starting an AIMD job from a transition state geometry with AIMD_INIT_VELOC=ZPE, the value of AIMD_KINETIC_ENERGY specifies the amount of energy to be put in the imaginary-frequency normal mode. The other modes are populated with their respective ZPEs and random velocity signs. The value of AIMD_KINETIC_ENERGY is given in kcal/mol in the fixed-point format: if AIMD_KINETIC_ENERGY=1234, the amount of kinetic energy in the TS mode is 123.4 kcal/mol.

There does not appear to be any control over the direction of the velocity vector for the TS normal mode (transition vector).

#3

Thank you for the response. If I just gave AIMD_KINETIC_ENERGY, TS will always go to the product. I tried to reverse the velocity or modify the code in order to obtain negative AIMD_KINETIC_ENERGY but they still went to the product. I am still trying to figure out how to obtain a trajectory from TS to the reactant.