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Difference between revisions of "NELMDL"

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Description: {{TAG|NELMDL}} specifies the number of non-selfconsistent steps at the beginning.
 
Description: {{TAG|NELMDL}} specifies the number of non-selfconsistent steps at the beginning.
 
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If one initializes the orbitals randomly the initial orbitals are far from anything reasonable. The resulting charge density is also 'nonsense'. Therefore it makes sense to keep the initial Hamiltonian, which corresponds to the superposition of atomic charge densities, fixed during the first few steps.
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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 required 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}}).
  
Choosing a 'delay' for starting the charge density update becomes essential in all cases where the SC-convergence is very bad (e.g. surfaces or molecules/clusters, chains). Without setting a delay VASP will probably not converge or at least the convergence speed is slowed down.
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Such a 'delay' is absolutely necessary. if the SCF-convergence is bad (e.g. surfaces or metallic clusters, low dimensional system). Without setting a delay VASP will most likely not converge or at least the convergence speed is slowed significantly.
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{{TAG|NELMDL}} might be set to a positive or negative value. A negative value employs the delay only in the first ionic step (usually the recommended option). A positive number means that a delay is applied after each ionic movement. This can improve the convergence speed in vasp.6 (see below), but is not recommended in vasp.5.
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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.
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VASP.6 special considerations:
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* 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. Using a positive {{TAG|NELMDL}} (i.e. delay in every ionic step), does not work reliably though 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 simulations or relaxations using {{TAG|ALGO}}=ALL or {{TAG|ALGO}}=DAMPED. 
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* For HF type calculations, if {{TAG|NELMDL}} is larger or equal 3, VASP will perform  {{TAG|NELMDL}} non-selfconsistent steps using the Davidson algorithm and a local Hamiltonian 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 for molecular dynamics simulations and relaxations using HF type Hamiltonians. Try to use {{TAG|ALGO}}=All and {{TAG|NELMDL}}=3, if you encounter convergence issues for relaxations and molecular dynamics simulations use HF type Hamiltonians.
  
{{TAG|NELMDL}} might be positive or negative. A positive number means that a delay is applied after each ionic movement (in general not a convenient option). A negative value results in a delay only for the start-configuration.
 
  
 
== Related Tags and Sections ==
 
== Related Tags and Sections ==

Revision as of 11:53, 12 August 2019

NELMDL = [integer] 

Default: NELMDL = -5 if ISTART=0, INIWAV=1, and IALGO=8
= -12 if ISTART=0, INIWAV=1, and IALGO=48

Description: NELMDL specifies the number of non-selfconsistent 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 required 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 bad (e.g. surfaces or metallic clusters, low dimensional system). Without setting 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 employs the delay only in the first ionic step (usually the recommended option). A positive number means that a delay is applied 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.

VASP.6 special considerations:

  • 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. Using a positive NELMDL (i.e. delay in every ionic step), does not work reliably though 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 simulations or relaxations using ALGO=ALL or ALGO=DAMPED.
  • For HF type calculations, if NELMDL is larger or equal 3, VASP will perform NELMDL non-selfconsistent steps using the Davidson algorithm and a local Hamiltonian 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 for molecular dynamics simulations and relaxations using HF type Hamiltonians. Try to use ALGO=All and NELMDL=3, if you encounter convergence issues for relaxations and molecular dynamics simulations use HF type Hamiltonians.


Related Tags and Sections

NELM, NELMIN

Examples that use this tag