MDALGO: Difference between revisions
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== {{TAG|MDALGO}}=13: Multiple Andersen thermostats == | == {{TAG|MDALGO}}=13: Multiple Andersen thermostats == | ||
<div id="multiAnderson"></div> | <div id="multiAnderson"></div> | ||
Up to three user-defined atomic subsystems may be coupled with independent [[Andersen thermostat|Andersen thermostats]]<ref name="Andersen80"/> ( | Up to three user-defined atomic subsystems may be coupled with independent [[Andersen thermostat|Andersen thermostats]]<ref name="Andersen80"/> ({{TAG|MDALGO}}=1). | ||
The {{FILE|POSCAR}} file must be organized such that the positions of atoms of subsystem ''i+1'' are defined after those for the subsystem ''i'', and the following | The {{FILE|POSCAR}} file must be organized such that the positions of atoms of subsystem ''i+1'' are defined after those for the subsystem ''i'', and the following tags must be set: {{TAG|NSUBSYS}}, {{TAG|TSUBSYS}}, and {{TAG|PSUBSYS}}. | ||
== Related tags and articles == | == Related tags and articles == | ||
Revision as of 07:29, 22 October 2025
MDALGO = 0 | 1 | 2 | 3 | 4 | 5 | 11 | 21 | 13
Default: MDALGO = 0
Description: Specifies the thermostat for MD calculations (in case IBRION = 0).
Mind: All options except MDALGO = 0 require a VASP executable which was compiled with the -Dtbdyn precompiler option enabled. This is usually the case because the option is present by default in all shipped makefile.include templates since VASP 5.4.4.
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MDALGO=1,2,3,4,5 can be applied in the context of
- standard molecular-dynamics calculations
- constrained molecular dynamics
- metadynamics calculations
- the slow-growth approach
- monitoring geometric parameters using the ICONST file
- Biased molecular dynamics
The main output file is the REPORT file.
MDALGO = 0 : Standard molecular dynamics
Selects a Nosé-Hoover thermostat which allows sampling the NVT ensemble at temperature TEBEG. The Nosé-Hoover thermostat requires an appropriate setting for SMASS. To sample an NVE ensemble set SMASS = -3 .
Deprecated: If possible, we recommend using one of the newer Nosé-Hoover thermostat implementations VASP provides (MDALGO = 2 or 4). While the results (ensemble averages) should be identical ,this variant comes with some drawbacks regarding post-processing: the atom coordinates in output files will always be wrapped back into the box if atoms cross the periodic boundaries. This makes it impossible to carry out certain analysis, e.g., computing the mean squared displacement (MSD).
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MDALGO = 1 : Andersen thermostat
The Andersen thermostat can be used to sample an NVT ensemble, which requires setting an appropriate value for ANDERSEN_PROB.
For ANDERSEN_PROB = 0.0 , the thermostat is inactive, such that the NVE ensemble is sampled. This is usually done after thermalization to a certain target temperature.
| Tip: Leave the value for TEBEG that was set in the thermalization. For TEBEG = <0.1 , some part of the code assumes it is used for structure optimization and not an MD run. |
MDALGO = 2 : Nosé-Hoover thermostat
The Nosé-Hoover thermostat is currently only available for the NVT ensemble. It requires setting an appropriate value for SMASS.
| Tip: The Nosé-Hoover thermostat is a special case of the Nosé-Hoover chain thermostat (MDALGO = 4 with NHC_NCHAINS = 1 ). The control tags for MDALGO = 4 may be more convenient to use than the older implementation (MDALGO = 2 ). |
MDALGO = 3 : Langevin thermostat
The Langevin thermostat is available for sampling an NVT ensemble as well as sampling an NpT ensemble.
- For an NVT ensemble, fix the cell shape and volume with ISIF = 2 and set an appropriate value for the friction coefficients for all species in the POSCAR file by means of the LANGEVIN_GAMMA tag.
- The Langevin dynamics in the NpT ensemble is calculated by the method of Parrinello and Rahman[1][2] combined with a Langevin thermostat.
- Set ISIF = 3 to allow for relaxation of the cell volume and shape. At the moment, dynamics with fixed volume+variable shape (ISIF=4) or fixed shape+variable volume (ISIF=7) are not available.
- Specify friction coefficients for all species in the POSCAR file by means of the LANGEVIN_GAMMA tag.
- Specify a separate set of friction coefficient for the lattice degrees-of-freedom using the LANGEVIN_GAMMA_L tag.
- Set a mass for the lattice degrees-of-freedom using the PMASS tag.
- Optionally, one may define an external pressure (in kB) by means of the PSTRESS tag.
Also see stochastic boundary conditions.
MDALGO = 4 : Nosé-Hoover chain thermostat
The Nosé-Hoover chain thermostat can be used to sample an NVT ensemble and requires selecting the number of thermostats in the chain via NHC_NCHAINS as well as choosing an appropriate setting for the thermostat parameter NHC_PERIOD.
MDALGO=5: Canonical sampling through velocity-rescaling (CSVR thermostat)
| Mind: This option is available as of VASP 6.4.3. |
The CSVR thermostat can be used to sample an NVT ensemble. It requires setting CSVR_PERIOD.
MDALGO=13: Multiple Andersen thermostats
Up to three user-defined atomic subsystems may be coupled with independent Andersen thermostats[3] (MDALGO=1). The POSCAR file must be organized such that the positions of atoms of subsystem i+1 are defined after those for the subsystem i, and the following tags must be set: NSUBSYS, TSUBSYS, and PSUBSYS.
Related tags and articles
IBRION, ISIF, SMASS, ANDERSEN_PROB, RANDOM_SEED, LBLUEOUT, SHAKETOL, SHAKEMAXITER, HILLS_H, HILLS_W, HILLS_BIN, INCREM, VALUE_MIN, VALUE_MAX, LANGEVIN_GAMMA, LANGEVIN_GAMMA_L, PMASS, NSUBSYS, TSUBSYS, PSUBSYS, ICONST, PENALTYPOT, HILLSPOT, REPORT