Hello colleagues! I found that my structure is not stable at the ground state, id. 0 Kelvin ( the crystal structure change before and after relaxation, but the energy is still negative) Thus I want to perform a relaxation at a different temperature. I have found some examples on the internet, but not a relaxation one.
This is my INCAR file, any orientation will be deeply appreciated.
ISTART = 0
ICHARG = 2
NSW = 1000
IBRION = 0
ISIF = 2
POTIM = 5
TEBEG =1200
TEEND =1200
ISYM = 0
SMASS= 0
LREAL = .FALSE.
MDALGO = 2
# precision parameters
PREC = Normal
EDIFF = 1E-6
# electronic relaxation
ALGO = FAST
ENCUT = 300
ISMEAR = 0
# output options
LWAVE = .FALSE. # write or don't write WAVECAR
LCHARG = .FALSE. # write or don't write CHG and CHGCAR
How do you know your system is not stable at T = 0 K? Your question mentions that the energy is negative - do you mean the total energy of the simulation cell? That is usually negative, it's not a problem.
To relax a structure in VASP at a given temperature other than 0 Kelvin, you can use the molecular dynamics (MD) method. This method simulates the motion of atoms in a system by solving the equations of motion for each atom based on the forces acting on it.
Here are the basic steps to perform an MD relaxation in VASP at a given temperature:
1. Set up an input file (INCAR) with the following parameters:
ISMEAR = -5 (to use Methfessel-Paxton smearing)
IBRION = 0 (to turn off ionic relaxation during MD)
NSW = (the number of MD steps you want to perform)
EDIFFG = (the convergence threshold for the forces)
IALGO = 48 (to use the fast inertial relaxation engine (FIRE) algorithm)
2. Set up a POSCAR file with your initial atomic coordinates and cell parameters.
3. Generate an initial velocity distribution for the atoms based on the desired temperature using the script "lvtot.sh" provided with VASP. This script generates a set of random velocities for the atoms in the system based on the specified temperature.
4. Run VASP with the above input files using the "md" command. This will perform an MD simulation at the specified temperature.
5. Analyze the output files to ensure that the system has equilibrated at the desired temperature. You can monitor the temperature of the system during the simulation using the "OUTCAR" file.
6. Repeat steps 4 and 5 if necessary until the system has equilibrated at the desired temperature.
7. Once the system has equilibrated, you can perform a relaxation of the atomic positions using the "IBRION=2" option. This will perform a relaxation of the atomic positions while keeping the cell parameters fixed.
Note that performing an MD simulation can be computationally expensive, so it is important to choose an appropriate simulation time and time step. You may need to adjust these parameters based on the size of your system and the desired temperature.
Thank you for your, time checking this Dr Philip James Hasnip . According to the phase diagram, my bcc structure is not stable at low temperatures, which could explain the failings of my relaxation at the ground state. (As I said, after relaxation the energy is negative but the crystal structure is not cubic anymore). Now I want to relax this structure at high temperatures, but I would appreciate any guidance on how to proceed.
It's not really meaningful to relax at a nonzero temperature. You've gotten a nice, detailed answer on how to first run an MD and then relax, if that's what you want to do. It's not really clear to me what your goal is, but I would guess you want to calculate the free energy of different structures at a given temperature to see what's most stable at that temperature. Alternatively, if you already know what structure you want to study, just force the system to stay in that structure by imposing the proper constraints on the cell (and possibly specific atoms).
Xi Zhu Thanks for the script, May I know why didn't you include the flag of temperature?
Matthew M. Montemore Thanks for your time. My goal is to relax a BCC structure while maintaining its cubic symmetry before and after relaxation. After checking the phase diagram, I found that based on the chemical composition of my system, BCC is only stable at high temperatures, while HCP is stable at low temperatures or the ground state. So I start to think that this is the reason why my structure after relaxation at 0 K is not cubic anymore. Thus I decided to perform calculations at High temperatures.
In my experience, don't trust anything before the 1980s when it comes to experimental phase diagrams; the error in their measurements was sufficiently large that a lot of things got mis-ascribed as cubic!
That said, if you are *certain* it is cubic, try ISIF = 7, which according to the vasp wiki allows the cell volume and ionic positions to relax, but not the shape of the cell.
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