NELMDL: Difference between revisions

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.

@@ Line 1: / Line 1: @@
 {{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-selfconsistent steps at the beginning.
+Description: {{TAG|NELMDL}} specifies the number of non-self-consistent steps at the beginning.
 ----
-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.
+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}}).
-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.
+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 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.
+{{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.
-== Related Tags and Sections ==
+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|NELMIN}},
+{{TAG|IALGO}}
+{{sc|NELMDL|Examples|Examples that use this tag}}
 ----
-[[The_VASP_Manual|Contents]]
-[[Category:INCAR]]
+[[Category:INCAR tag]][[Category:Electronic minimization]]