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{{TAGDEF|VCAIMAGES|[real]|-1}}
{{TAGDEF|VCAIMAGES|[real]|-1}}


Description: The tag {{TAG|VCAIMAGES}} allows to perform thermodynamic coupling constant integrations. To achive this two molecular dynamics simulations are performed with e.g. different {{TAG|POTCAR}} or {{TAG|KPOINT}} files and force averaging between the two calculations is done.
Description: The tag {{TAG|VCAIMAGES}} allows to perform thermodynamic integrations (TI); it defines the coupling parameter λ.  
----
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The tag {{TAG|VCAIMAGES}} internally splits the available nodes into two groups, and each group
{{TAG|VCAIMAGES}} allows two molecular dynamics (MD) simulations to be performed with e.g. different {{TAG|POTCAR}} or {{TAG|KPOINTS}} files or different exchange-correlation functionals, and averages the energies and forces between the two calculations. This is known as thermodynamic integration (TI) {{cite|dorner:PRL:2018}}.
then performs an independent VASP calculation (this implies {{TAG|VCAIMAGES}} only works in the MPI version).
This behavior is implemented in the same way as
the nudged elastic band method described under the tag {{TAG|IMAGES}}. As opposed to the nudged
elastic band method, VASP creates always two images by setting the tag {{TAG|IMAGES}}=2 internally.
The two calculations are performed in the subdirectories 01 and 02 (as opposed to the nudged elastic
band method, the subdirectories 00 and 03  are not required). A precise description of the file
reading and writing is given below.


The two VASP calculations are essentially performed independently in the subdirectories 01 and 02.
The tag {{TAG|VCAIMAGES}} internally splits the available nodes into two groups, and each group then performs an independent VASP calculation (this implies {{TAG|VCAIMAGES}} only works in the MPI version). This behavior is implemented in the same way as the [[Nudged elastic bands | nudged elastic band method]] (NEB) described under the tag {{TAG|IMAGES}}. As opposed to NEB, only two images are created ({{TAG|IMAGES}}=2 is set internally). The two calculations are performed in subdirectories <code>01</code> and <code>02</code> (<code>00</code> and <code>03</code> are not required, in contrast to NEB).
Only the forces and energies of the two calculations are averaged according the to tag {{TAG|VCAIMAGES}}.
Specificially, the value supplied in the tag {{TAG|VCAIMAGES}} determines the weight of the calculations
performed in the subdirectory 01. The weight of the second image is 1-{{TAG|VCAIMAGES}}. The averaging
is performed after self-consistency has been reached and affects the final total energy as well as
the forces. Since the energies and forces are averaged, the trajectories generated by the two simulations
will be exactly identical. Make sure though, that the initial {{TAG|POSCAR}} files in the two subdirectories
are exactly identical. The averaged energies can be found in the {{TAG|OUTCAR}} file after the lines
"ENERGY OF THE ELECTRON-ION-THERMOSTAT SYSTEM (eV)", as well as in the file {{TAG|OSZICAR}}
(in the lines writing the free energy  "F="). This makes scanning the {{TAG|OSZICAR}} for the
required energies of the individual calculations somewhat akward.


=== Description of reading a writing during the calculation ===
The two calculations are performed essentially independently in subdirectories <code>01</code> and <code>02</code>. The forces, energies, and the stress tensor of the two calculations are averaged according to the coupling parameter supplied by {{TAG|VCAIMAGES}}. Specifically, the value supplied in the tag {{TAG|VCAIMAGES}} determines the weight of the calculations performed in subdirectory <code>01</code>. The weight of the second image is 1-{{TAG|VCAIMAGES}}. After self-consistency has been reach for both calculations, the energies and forces are averaged, affecting the final total energy as well as the forces. This ensures that the trajectories for the two MD simulations are identical.
{{NB|important|Make sure that the initial {{FILE|POSCAR}} is identical in both subdirectories.}}


File handling: When VASP is started it reads the file {{TAG|INCAR}} in the root directory.
=== Finding the energies ===
The following tags are also read from the root {{TAG|INCAR}} file:
The averaged energies can be found in the {{TAG|OUTCAR}} file after the lines
{{TAG|NCORE_IN_IMAGES1}}, {{TAG|IMAGES}}, {{TAG|KIMAGES}}, {{TAG|FOURORBIT}}, {{TAG|KPAR}}, {{TAG|NCORE}},
<code>ENERGY OF THE ELECTRON-ION-THERMOSTAT SYSTEM (eV)</code>, as well as in the file {{TAG|OSZICAR}}
{{TAG|NCORES_PER_BAND}}, {{TAG|NPAR}}, {{TAG|NCSHMEM}}. Subsequently, VASP
(in the lines writing the free energy <code>F=</code>). This can make looking for the energies of the individual calculation awkward. You can find these under <code>FREE ENERGIE OF THE ION-ELECTRON SYSTEM (eV)</code> in the {{FILE|OUTCAR}} file for a DFT calculation (They are under <code>ML FREE ENERGIE OF THE ION-ELECTRON SYSTEM (eV)</code> for a machine-learned force field (MLFF)). 
splits the MPI communicator into two  subgroups and continues reading from the {{TAG|INCAR}}
{{NB|important|In some cases it might be desirable to use a different number of cores for
file in the subdirectories 01 and 02. If the INCAR files are not present in the subdirectories 01 or 02,
the first image and the second image. E.g., when the thermodynamic integration is performed from a coarse to a dense k-point grid, or from a cheap
VASP will continue reading from the root INCAR file. The same logic is used from the files {{TAG|KPOINTS}} and {{TAG|POTCAR}}:
to an expensive exchange-correlation functional. To set the number of cores in the first image the tag {{TAG|NCORE_IN_IMAGE1}} has to be set. The second image then
if they exist in the subdirectories, they will be read from the subdirectories 01 and 02, if they
contains the remaining cores.}}
are missing the files are read from the root directory. The {{TAG|POSCAR}} and all other input files
The usage of this tag is also explained in the supplementary information of reference {{cite|dorner:PRL:2018}}.
are always read from the subdirectories 01 and 02. All output files (including {{TAG|OUTCAR}} and {{TAG|OSZICAR}})
are always written to the subdirectories 01 and 02.  


== Related tags and articles ==
{{TAG|NCORE_IN_IMAGE1}}, {{TAG|SCALEE}}, {{TAG|IMAGES}}


Special handling: In some cases it might be desirable to use a different number of cores for
== References ==
the first image and the second image. For instance, this might be expedient for calculations
[[Category:INCAR tag]][[Category:Advanced molecular-dynamics sampling]]
where thermodynmic integration is performed from a coarse to a dense k-point grid.
To set the number of cores in the first image the tag {{TAG|NCORE_IN_IMAGE1}} has to be set. The second image then
contains the remaining cores.
 
== Related Tags and Sections ==
{{TAG| NCORE_IN_IMAGE1| IMAGES}}
 
----
 
[[Category:INCAR]][[Category:Molecular Dynamics]][[Category:Thermodynamic integration]]

Latest revision as of 11:37, 20 March 2026

VCAIMAGES = [real]
Default: VCAIMAGES = -1 

Description: The tag VCAIMAGES allows to perform thermodynamic integrations (TI); it defines the coupling parameter λ.

VCAIMAGES allows two molecular dynamics (MD) simulations to be performed with e.g. different POTCAR or KPOINTS files or different exchange-correlation functionals, and averages the energies and forces between the two calculations. This is known as thermodynamic integration (TI) [1].

The tag VCAIMAGES internally splits the available nodes into two groups, and each group then performs an independent VASP calculation (this implies VCAIMAGES only works in the MPI version). This behavior is implemented in the same way as the nudged elastic band method (NEB) described under the tag IMAGES. As opposed to NEB, only two images are created (IMAGES=2 is set internally). The two calculations are performed in subdirectories 01 and 02 (00 and 03 are not required, in contrast to NEB).

Description of reading a writing during the calculation

The two calculations are performed essentially independently in subdirectories 01 and 02. The forces, energies, and the stress tensor of the two calculations are averaged according to the coupling parameter supplied by VCAIMAGES. Specifically, the value supplied in the tag VCAIMAGES determines the weight of the calculations performed in subdirectory 01. The weight of the second image is 1-VCAIMAGES. After self-consistency has been reach for both calculations, the energies and forces are averaged, affecting the final total energy as well as the forces. This ensures that the trajectories for the two MD simulations are identical.

Important: Make sure that the initial POSCAR is identical in both subdirectories.

Finding the energies

The averaged energies can be found in the OUTCAR file after the lines ENERGY OF THE ELECTRON-ION-THERMOSTAT SYSTEM (eV), as well as in the file OSZICAR (in the lines writing the free energy F=). This can make looking for the energies of the individual calculation awkward. You can find these under FREE ENERGIE OF THE ION-ELECTRON SYSTEM (eV) in the OUTCAR file for a DFT calculation (They are under ML FREE ENERGIE OF THE ION-ELECTRON SYSTEM (eV) for a machine-learned force field (MLFF)).

Important: In some cases it might be desirable to use a different number of cores for

the first image and the second image. E.g., when the thermodynamic integration is performed from a coarse to a dense k-point grid, or from a cheap to an expensive exchange-correlation functional. To set the number of cores in the first image the tag NCORE_IN_IMAGE1 has to be set. The second image then contains the remaining cores.

The usage of this tag is also explained in the supplementary information of reference [1].

Related tags and articles

NCORE_IN_IMAGE1, SCALEE, IMAGES

References

  1. ↑ a b F. Dorner, Z. Sukurma, C. Dellago, and G. Kresse, Phys. Rev. Lett. 121, 195701 (2018).
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