Hello all,
I am putting in practice RESPA calculations, and I need some advice with some details that I found.
My system is liquid Deep Eutectic Solvent, which I run a long MD with GGA functional, then it is converged for GGA at least. I start from there to run the MD. I ran 3 MD: BLYP, B3LYP and B3LYP(BLYP)-RESPA. All of them with 0.5 fs of time step, and a frequency of 5 for the RESPA. I attach the input file for the RESPA.
I run these 3 MDs to compare and check the quality of the RESPA MD in a "realistic" system, instead over a single molecule MD. First of all I try to check the potential energy, and all it is ok, RESPA obtains pretty good values. (see Potential.eps, all energy values are relative to the first snapshot energy of each dynamic).
But the conserved quantity (Kinetic+Potential) is not conserved in the case of RESPA, but in the pure B3LYP calculation there is no problem about it. (see Conserved.eps). There is a tendency to get lower energies along the MD (~5mHa for 20 fs)). I do not know if this behavior is "normal" in RESPA calculations with respect to the standard hybrid calculations, the papers do not say anything about that. Searching for the error, I plot the kinetic energy (Kinetic.eps), which shows that the RESPA calculation evolves "losing" kinetic energy with respect to the standard B3LYP MD... Finally to see the values of mismatch between both MD, I compute the delta kinetic and potential energy (Delta.eps), it is pretty clear that potential energy difference is very low (<2 mHa) and it is sometime positive and sometime negative which means there is no bias in the potential computation, but for kinetic the differences is always negative and ~4 mHa, which explains that conserved quantity is not conserved.
After that, I have some questions...
1- Is it normal this behavior on kinetic energy for RESPA calculations?
2- How can I improve the MD to get a conserved total energy in this case? Because for the standard B3LYP MD there is no problem with it.
3- Are there any criteria that should be tight in the calculation? I calculate potential energy for the first snapshot with EPS_SCHWARZ 1E-12 and SCREENING FALSE, and the energy decreases just 0.3 mHa, and the energy convergence is about 1E-6...
4- Could I be calm about this and proceed to a longer dynamic without worrying?
Usually I run calculations by trial and error to understand the problems and improve the calculations, but these calculations are no cheaper and I will need one or two weeks to search with a couple of variotions on the keywords, that is beacuse I am here.
Regards!