strange energy in MD simulation

[bo_gao]

Dear all,
Now I'm performing the MD simulations under the temperature increasing from 0 to 400K. I found that the calculated energy evolution with time step is strange. The energy is severely decreased at the initial stage.

free energy TOTEN = -2475.95578729 eV
free energy TOTEN = -2475.96164896 eV
free energy TOTEN = -2475.98039764 eV
free energy TOTEN = -2476.01186756 eV
free energy TOTEN = -2476.05315420 eV
free energy TOTEN = -2476.10422642 eV
free energy TOTEN = -2476.16466461 eV
free energy TOTEN = -2476.23544330 eV
free energy TOTEN = -2476.31572044 eV
free energy TOTEN = -2476.40435812 eV
free energy TOTEN = -2476.50098890 eV
free energy TOTEN = -2476.60463008 eV
free energy TOTEN = -2476.72436786 eV
free energy TOTEN = -2476.85121651 eV
free energy TOTEN = -2476.98585512 eV
free energy TOTEN = -2477.12785025 eV
free energy TOTEN = -2477.28693910 eV
free energy TOTEN = -2477.45351982 eV
free energy TOTEN = -2477.62718308 eV
free energy TOTEN = -2477.81671685 eV
free energy TOTEN = -2478.01603236 eV
free energy TOTEN = -2478.22502882 eV
free energy TOTEN = -2478.44226617 eV
free energy TOTEN = -2478.67230703 eV
free energy TOTEN = -2478.91136933 eV
free energy TOTEN = -2479.15888329 eV
free energy TOTEN = -2479.41643393 eV
free energy TOTEN = -2479.68408841 eV
free energy TOTEN = -2479.96100479 eV
free energy TOTEN = -2480.24598856 eV
free energy TOTEN = -2480.54524711 eV
free energy TOTEN = -2480.85208886 eV
free energy TOTEN = -2481.16760692 eV
free energy TOTEN = -2481.49332460 eV
free energy TOTEN = -2481.82673370 eV
free energy TOTEN = -2482.17449121 eV
free energy TOTEN = -2482.53118432 eV
free energy TOTEN = -2482.89663781 eV
free energy TOTEN = -2483.27292110 eV
free energy TOTEN = -2483.66079968 eV
free energy TOTEN = -2484.06023605 eV
free energy TOTEN = -2484.46907584 eV
free energy TOTEN = -2484.89111603 eV
free energy TOTEN = -2485.32169484 eV
free energy TOTEN = -2485.76205409 eV
free energy TOTEN = -2486.21069262 eV
free energy TOTEN = -2486.67398117 eV
free energy TOTEN = -2487.14491070 eV
free energy TOTEN = -2487.62343859 eV
free energy TOTEN = -2488.11265541 eV
free energy TOTEN = -2488.61185266 eV
free energy TOTEN = -2489.12075089 eV
free energy TOTEN = -2489.63820062 eV
free energy TOTEN = -2490.16975597 eV
free energy TOTEN = -2490.71140867 eV
free energy TOTEN = -2491.26301656 eV
free energy TOTEN = -2491.82403070 eV
free energy TOTEN = -2492.39951321 eV
free energy TOTEN = -2492.98469304 eV
free energy TOTEN = -2493.57901038 eV
free energy TOTEN = -2494.18522337 eV
free energy TOTEN = -2494.80175715 eV
free energy TOTEN = -2495.42720380 eV
free energy TOTEN = -2496.06359678 eV

Here the initial structure in the MD simulation has already been geometrically optimized.

Then I extracted the snapshot structures in the MD simulation and performed the individual SCF calculation. I found that the calculated energy in the individual SCF calculation is quite different (above 1eV) from that in the MD simulation.

This is INCAR used in the MD simulation:
SYSTEM = opt-2
PREC = Normal
ENCUT = 450
EDIFF = 1e-4
#EDIFFG = 5e-4
IBRION = 0
ISIF = 2
NSW = 2000
NELM = 600
ISMEAR = 0
SIGMA = 0.05
POTIM = 2
LCHARG = FALSE
LWAVE = FALSE
ISTART = 0
#AMIN = 0.01
LREAL = Auto
KSPACING = 0.8
NPAR = 12
MDALGO = 2
TEBEG = 0
TEEND = 400
SMASS = 2
NWRITE = 1
NPAR = 12
#ALGO = Fast

I hope someone can provide the answer to this problem.

[ferenc_karsai]

Please also post your POSCAR, POTCAR, KPOINTS and OUTCAR files.

[bo_gao]

Thank you. I attached the INCAR, POTCAR, POSCAR and OUTCAR files. Here I used the gamma-only version VASP, so the KPOINTS file was not set.

[ferenc_karsai]

So I've looked at your calculation.
When one looks at the total energy including kinetic energy and Nose-Hoover contributions (grep "total energy ETOTAL" OUTCAR), one would expect that this quantity is conserved at constant temperature. Of course you have a temperature ramp so it is expected that this value would slightly change. But in your calculation the energy changes within 200 by more than 200eV. This is too much. By playing around we found that the starting temperature TEBEG=0 is causing this. By doing the same calculation but setting TEBEG to a small non-zero value the problem seems to be solved. I've ran the same calculation but with TEBEG=10 and the changes of ETOTAL within the first 200 steps goes down to roughly 1eV.

So please don't use 0K as a starting temperature, but some small value >0.

[bo_gao]

It works. Thank you very much!