Restarting finite differences calculations

It is possible to restart finite difference calculations using IBRION = 6 and CHECKPOINT_FD. The displacements are written to a vaspcheckfd.h5 file. For details of a general finite difference calculation, see the phonons from finite differences. Here, we will concern ourselves with restarting and splitting finite difference calculations.

Mind: This can only be done using IBRION = 6. We recommend using this generally over IBRION = 5.

There are several options for the CHECKPOINT_FD tag. The default is CHECKPOINT_FD = RESET, which creates a new vaspcheckfd.h5 file and updates the file during the calculation after each displacement. CHECKPOINT_FD = CONTINUE continues from the last completed displacement and CHECKPOINT_FD = PREPARE creates the displacements and stops after the electronic minimization for the equilibrium structure. CHECKPOINT_FD = SINGLE is used to run individual displacements.

We will describe the restart procedure and splitting a calculation below. As an example, we take a 3x3x1 graphene supercell POSCAR file from the phonon tutorials.

Click to see POSCAR and INCAR

C18
1.0
   7.3521657209830806    0.0000000000000000    0.0000000000000000
  -3.6760828604915403    6.3671622872044793    0.0000000000000000
   0.0000000000000000    0.0000000000000000    8.0000000000000000
C
18
direct
   0.1111111111111133    0.2222222222222200    0.0000000000000000 C
   0.1111111111111133    0.5555555555555532    0.0000000000000000 C
   0.1111111111111133    0.8888888888888866    0.0000000000000000 C
   0.4444444444444466    0.2222222222222200    0.0000000000000000 C
   0.4444444444444466    0.5555555555555532    0.0000000000000000 C
   0.4444444444444466    0.8888888888888866    0.0000000000000000 C
   0.7777777777777801    0.2222222222222200    0.0000000000000000 C
   0.7777777777777799    0.5555555555555532    0.0000000000000000 C
   0.7777777777777799    0.8888888888888866    0.0000000000000000 C
   0.2222222222222200    0.1111111111111133    0.0000000000000000 C
   0.2222222222222200    0.4444444444444466    0.0000000000000000 C
   0.2222222222222199    0.7777777777777799    0.0000000000000000 C
   0.5555555555555532    0.1111111111111133    0.0000000000000000 C
   0.5555555555555534    0.4444444444444466    0.0000000000000000 C
   0.5555555555555532    0.7777777777777799    0.0000000000000000 C
   0.8888888888888866    0.1111111111111133    0.0000000000000000 C
   0.8888888888888866    0.4444444444444466    0.0000000000000000 C
   0.8888888888888866    0.7777777777777799    0.0000000000000000 C

along with a 4x4x1 k-mesh in our KPOINTS file:

K points
 0
Gamma
4  4  1
0  0  0

and PAW C_s 04May1998 POTCAR.

The following INCAR file with modifications will be used thoughout:

SYSTEM = graphene
ENCUT = 400

# electronic
PREC = Accurate
NELMIN = 5
EDIFF = 1e-8
ISMEAR = -1
SIGMA = 0.2
LREAL = .FALSE.
LWAVE = .FALSE.
LCHARG = .FALSE.

# ionic (finite differences)
IBRION = 6
POTIM = 0.015

Restarting a finite difference calculation

The calculation failed after a certain point or has accidentally been cancelled:

DAV:   9    -0.181350430125E+03   -0.25600E-06   -0.38452E-08   848   0.118E-03    0.774E-04
DAV:  10    -0.181350430581E+03   -0.45554E-06   -0.20686E-08   864   0.738E-04    0.208E-04
DAV:  11    -0.181350430676E+03   -0.94857E-07   -0.22346E-09   704   0.354E-04    0.207E-04
srun: Job step aborted: Waiting up to 32 seconds for job step to finish.
slurmstepd-test01: error: *** JOB 254054 ON test01 CANCELLED AT 2026-01-19T16:25:49 ***
slurmstepd-test01: error: *** STEP 254054.0 ON test01 CANCELLED AT 2026-01-19T16:25:49 ***

In the directory, you will see the vaspcheckfd.h5 file. It contains the displacement calculations that have been completed up to the point of the crash:

h5ls vaspcheckfd.h5

data-1                   Group
displacements            Group
symmetry                 Group

Restart the calculation by adding CHECKPOINT_FD = CONTINUE to the INCAR:

SYSTEM = graphene
ENCUT = 400

# electronic
PREC = Accurate
NELMIN = 5
EDIFF = 1e-8
ISMEAR = -1
SIGMA = 0.2
LREAL = .FALSE.
LWAVE = .FALSE.
LCHARG = .FALSE.

# ionic (finite differences)
IBRION = 6
POTIM = 0.015
CHECKPOINT_FD = CONTINUE

and resubmit your calculation in the directory. The finite differences calculation will continue after an SCF step has been done and the stdout reads

Continuing from previous run

The calculation finishes as normal when all displacements have been completed and the phonon modes calculated:

h5ls vaspcheckfd.h5

data-1                   Group
data-2                   Group
data-3                   Group
data-4                   Group
displacements            Group
symmetry                 Group

Splitting a finite difference calculation

For large structures, it may be easier to split the displacements into separate calculations. This is done in three steps:

1. Preparing the separate displacements - CHECKPOINT_FD = PREPARE
2. Single displacement calculations - CHECKPOINT_FD = SINGLE
3. Collected finite differences - CHECKPOINT_FD = CONTINUE

1. Preparing the displacements

The displacements can be prepared using the CHECKPOINT_FD = PREPARE tag:

SYSTEM = graphene
ENCUT = 400

# electronic
PREC = Accurate
NELMIN = 5
EDIFF = 1e-8
ISMEAR = -1
SIGMA = 0.2
LREAL = .FALSE.
LWAVE = .FALSE.

# ionic (finite differences)
IBRION = 6
POTIM = 0.015
CHECKPOINT_FD = PREPARE

This creates CONTCAR_disp-N files containing each of the displacements in the parent directory and you can see the following in the stdout:

Creating CONTCAR files for finite difference displacements

and in the vaspcheckfd.h5 file:

h5ls vaspcheckfd.h5

metadata                 Group
subdir_prefix            Dataset {SCALAR}
total_count              Dataset {SCALAR}

2. Single displacement calculations

Create directories disp-N for each of the CONTCAR_disp-N file and run the calculations separately. To this end, copy all restart files in the subdirectory, rename CONTCAR_disp-N to POSCAR, set CHECKPOINT_FD = SINGLE in the INCAR file and run the calculation:

max=$(printf "%s\n" CONTCAR_disp-* | sed 's/.*-//' | sort -n | tail -1)

for i in $(seq 1 $max); do
  mkdir -p disp-$i
  cp CONTCAR_disp-$i disp-$i/POSCAR
  cp INCAR POTCAR KPOINTS vasp.run disp-$i/
  sed -i 's/PREPARE/SINGLE/g' disp-$i/INCAR
  echo """
NCORE = 4 
ICHARG = 1
LCHARG = F
""" >> disp-$i/INCAR
  ln -s ../CHGCAR disp-$i/CHGCAR
done

Each INCAR file in the directories will then look like:

SYSTEM = graphene
ENCUT = 400

# electronic
PREC = Accurate
NELMIN = 5
EDIFF = 1e-8
ISMEAR = -1
SIGMA = 0.2
LREAL = .FALSE.
LWAVE = .FALSE.

# ionic (finite differences)
IBRION = 6
POTIM = 0.015
CHECKPOINT_FD = SINGLE
NCORE = 4
ICHARG = 1
LCHARG = F

Important: You can set different NCORE settings in these calculations, offering parallelization that is not otherwise possible for finite differences.

Tip: It is optional to restart from WAVECAR or CHGCAR files.

In each subdirectory, you can see that a single displacement has been recorded in the stdout:

Computing single independent displacement for finite differences

and to the vaspcheckfd.h5 file:

h5ls vaspcheckfd.h5
 
data-1                   Group

3. Collected finite differences

Return to the parent directory and combine these separate displacements into one finite difference calculation using the CHECKPOINT_FD = CONTINUE tag:

SYSTEM = graphene
ENCUT = 400

# electronic
PREC = Accurate
NELMIN = 5
EDIFF = 1e-8
ISMEAR = -1
SIGMA = 0.2
LREAL = .FALSE.
LWAVE = .FALSE.
LCHARG = .FALSE.

# ionic (finite differences)
IBRION = 6
POTIM = 0.015
CHECKPOINT_FD = CONTINUE

Warning: It is possible to combine the finite difference calculations in a different directory but you must include the vaspcheckfd.h5 from the prepare step (as it contains the name of the subdirectories as metadata) and the subdirectories including the corresponding vaspcheckfd.h5.

Running this calculation, you can see that each of the single displacements are combined into one calculation in the stdout:

 Combining displacements from subdirectories

and vaspcheckfd.h5 file:

h5ls vaspcheckfd.h5

metadata                 Group
subdir_prefix            Dataset {SCALAR}
total_count              Dataset {SCALAR}

The information about the computed phonon modes is written to stdout and OUTCAR file below Eigenvectors and eigenvalues of the dynamical matrix in the same way as in phonons from finite differences.

Practical hints

• The phonon frequencies will differ slightly between one run and the split calculation, on the order of the 4th or 5th significant figure. We do not expect this to be significant.
• You can use this method for any electron-phonon calculations.
• You can set different NCORE settings in these calculations, offering parallelization that is not otherwise available for finite differences.
• Make sure to check that you are using the correct vaspcheckfd.h5 file with each calculation, particularly the split calculation. If data is read from an inappropriate vaspcheckfd.h5 file, you will see it with the following warning:

 -----------------------------------------------------------------------------
|                                                                             |
|     EEEEEEE  RRRRRR   RRRRRR   OOOOOOO  RRRRRR      ###     ###     ###     |
|     E        R     R  R     R  O     O  R     R     ###     ###     ###     |
|     E        R     R  R     R  O     O  R     R     ###     ###     ###     |
|     EEEEE    RRRRRR   RRRRRR   O     O  RRRRRR       #       #       #      |
|     E        R   R    R   R    O     O  R   R                               |
|     E        R    R   R    R   O     O  R    R      ###     ###     ###     |
|     EEEEEEE  R     R  R     R  OOOOOOO  R     R     ###     ###     ###     |
|                                                                             |
|     Checkpoint file vaspcheckfd.h5 is incompatible: NIONS mismatch          |
|     (file=54, current=128). Ensure that your calculational settings are     |
|     identical between runs. In particular, check POSCAR, KPOINTS,           |
|     POTCAR, and INCAR files.                                                |
|                                                                             |
|       ---->  I REFUSE TO CONTINUE WITH THIS SICK JOB ... BYE!!! <----       |
|                                                                             |
 -----------------------------------------------------------------------------

• We recommend using the CHGCAR file from the parent directory of the split calculation for the individual displacements to speed up the calculation. Link this with ln -s ../CHGCAR, set LCHARG = .FALSE. so that the original charge density is not overwritten. Also set ICHARG = 1 so that the charge density is used, rather than starting from scratch.

Related tags and sections

CHECKPOINT_FD, vaspcheckfd.h5, IBRION, CONTCAR_disp-N

Phonons from finite differences