NELMDL: Difference between revisions

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{{TAGDEF|NELMDL|[integer]}}
{{TAGDEF|NELMDL|[integer]}}
{{DEF|NELMDL|-5|if {{TAG|ISTART}}=0, {{TAG|INIWAV}}=1, and {{TAG|IALGO}}=8|-12|if {{TAG|ISTART}}=0, {{TAG|INIWAV}}=1, and {{TAG|IALGO}}=48}}
{{DEF|NELMDL|-12|if {{TAG|ISTART}}{{=}}0, {{TAG|INIWAV}}{{=}}1, and {{TAG|IALGO}}{{=}}48 or {{TAG|IALGO}}{{=}}50|0|if {{TAG|WAVECAR}} is present|-5| else}}
 
Description: {{TAG|NELMDL}} specifies the number of non-self-consistent steps at the beginning.
----
If the orbitals are initialized using a random number generator (the default in VASP),  the initial orbitals are usually unreasonable and the iterative matrix diagonalization will require 5-10 steps to obtain reasonable orbitals. The charge density corresponding to the initial orbitals is also, at best, erratic. It is hence advisable to perform a few electronic steps while keeping the initial Hamiltonian fixed. This initial Hamiltonian is usually determined from a superposition of atomic charge densities (see {{TAG|ICHARG}}).
 
Such a 'delay' is absolutely necessary if the SCF-convergence is slow and problematic (e.g. for surfaces or metallic clusters, low dimensional system). Without a delay, VASP will most likely not converge, or at least the convergence speed is slowed significantly.
 
{{TAG|NELMDL}} might be set to a positive or negative value. A negative value means that the delay is only performed in the first ionic step (usually the recommended option). A positive number means that a delay is employed after each ionic movement. This can improve the convergence speed in VASP.6 (see below) but is not recommended in VASP.5.
 
For calculations using a direct minimization of the Hamiltonian ({{TAG|ALGO}}=ALL  or {{TAG|ALGO}}=DAMPED), the Davidson algorithm is used during the delay phase and the Hamiltonian is kept fixed during these steps.
 
Special considerations for VASP.6:
 
* For calculations using a direct minimization of the Hamiltonian ({{TAG|ALGO}}=ALL or {{TAG|ALGO}}=DAMPED): if {{TAG|NELMDL}} is set, the Davidson algorithm is used in the first {{TAG|NELMDL}} steps as described above. Using a positive {{TAG|NELMDL}} (i.e. delay in every ionic step) does not work reliably in VASP.5, due to issues in the orbital and charge density prediction. In VASP.6, using {{TAG|NELMDL}}=1 (or {{TAG|NELMDL}}=2) and direct minimization often improves the stability and efficiency of molecular dynamics simulations or relaxations ({{TAG|ALGO}}=ALL or {{TAG|ALGO}}=DAMPED). Note, however, that this might require one to prepare a reasonable {{TAG|WAVECAR}} file since {{TAG|NELMDL}} =1/2 might not suffice to obtain a reasonable set of orbitals from the initial random numbers.
 
* For HF-type calculations, if {{TAG|NELMDL}} is larger or equal to 3, VASP will perform  {{TAG|NELMDL}} non-selfconsistent steps using the Davidson algorithm, and a local Hamiltonian is calculated using the semi-local DFT functional corresponding to the chosen hybrid functional (i.e. PBE for HSE and PBE0). This is expedient if the ions move by a large distance between the ionic steps. Setting  {{TAG|NELMDL}} =3 can thus improve the stability and performance during relaxations using HF-type Hamiltonians. Try to use {{TAG|ALGO}}=All and {{TAG|NELMDL}}=3 if you encounter convergence issues during relaxations using HF-type Hamiltonians.
 
== Related tags and articles ==
{{TAG|NELM}},
{{TAG|NELMIN}},
{{TAG|IALGO}}
 
{{sc|NELMDL|Examples|Examples that use this tag}}
----
 
[[Category:INCAR tag]][[Category:Electronic minimization]]

Latest revision as of 13:44, 27 November 2023

NELMDL = [integer] 

Default: NELMDL = -12 if ISTART=0, INIWAV=1, and IALGO=48 or IALGO=50
= 0 if WAVECAR is present
= -5 else

Description: NELMDL specifies the number of non-self-consistent steps at the beginning.


If the orbitals are initialized using a random number generator (the default in VASP), the initial orbitals are usually unreasonable and the iterative matrix diagonalization will require 5-10 steps to obtain reasonable orbitals. The charge density corresponding to the initial orbitals is also, at best, erratic. It is hence advisable to perform a few electronic steps while keeping the initial Hamiltonian fixed. This initial Hamiltonian is usually determined from a superposition of atomic charge densities (see ICHARG).

Such a 'delay' is absolutely necessary if the SCF-convergence is slow and problematic (e.g. for surfaces or metallic clusters, low dimensional system). Without a delay, VASP will most likely not converge, or at least the convergence speed is slowed significantly.

NELMDL might be set to a positive or negative value. A negative value means that the delay is only performed in the first ionic step (usually the recommended option). A positive number means that a delay is employed after each ionic movement. This can improve the convergence speed in VASP.6 (see below) but is not recommended in VASP.5.

For calculations using a direct minimization of the Hamiltonian (ALGO=ALL or ALGO=DAMPED), the Davidson algorithm is used during the delay phase and the Hamiltonian is kept fixed during these steps.

Special considerations for VASP.6:

  • For calculations using a direct minimization of the Hamiltonian (ALGO=ALL or ALGO=DAMPED): if NELMDL is set, the Davidson algorithm is used in the first NELMDL steps as described above. Using a positive NELMDL (i.e. delay in every ionic step) does not work reliably in VASP.5, due to issues in the orbital and charge density prediction. In VASP.6, using NELMDL=1 (or NELMDL=2) and direct minimization often improves the stability and efficiency of molecular dynamics simulations or relaxations (ALGO=ALL or ALGO=DAMPED). Note, however, that this might require one to prepare a reasonable WAVECAR file since NELMDL =1/2 might not suffice to obtain a reasonable set of orbitals from the initial random numbers.
  • For HF-type calculations, if NELMDL is larger or equal to 3, VASP will perform NELMDL non-selfconsistent steps using the Davidson algorithm, and a local Hamiltonian is calculated using the semi-local DFT functional corresponding to the chosen hybrid functional (i.e. PBE for HSE and PBE0). This is expedient if the ions move by a large distance between the ionic steps. Setting NELMDL =3 can thus improve the stability and performance during relaxations using HF-type Hamiltonians. Try to use ALGO=All and NELMDL=3 if you encounter convergence issues during relaxations using HF-type Hamiltonians.

Related tags and articles

NELM, NELMIN, IALGO

Examples that use this tag