Volume relaxation - Revision history

Schlipf at 13:45, 7 March 2025

2025-03-07T13:45:35Z

← Older revision		Revision as of 13:45, 7 March 2025
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	</li>		</li>
	</ol>		</ol>
	~~[[Category:PAW]][[Category:Volume relaxation]][[Category:Howto]]~~

	==References==		==References==

			[[Category:Projector-augmented-wave method]][[Category:Howto]]

Csheldon: /* How to correct */

2024-09-09T09:26:49Z

How to correct

← Older revision		Revision as of 09:26, 9 September 2024
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	{{TAG\|ISIF}} = 7 ; {{TAG\|IBRION}} = 2; {{TAG\|NSW}} = 20		{{TAG\|ISIF}} = 7 ; {{TAG\|IBRION}} = 2; {{TAG\|NSW}} = 20
	{{TAG\|ISTART}} = 1 # this keeps that energy cut-off constant, while updating the plane wave basis for the new cell volume		{{TAG\|ISTART}} = 1 # this keeps that energy cut-off constant, while updating the plane wave basis for the new cell volume
			{{NB\|mind\| In this procedure, it is important to set {{TAG\|ISTART}} {{=}} 1 so that the basis is updated and the energy cut-off remains constant. If {{TAG\|ISTART}} {{=}} 2, the basis remains constant, which will also keep the artificial, Pulay, stress, i.e. it is an exact restart of the calculation in Step 1, with identically poor settings. }}

	=== Step 3. ===		=== Step 3. ===

Csheldon: /* 1. Volume relaxation */

2024-09-09T08:45:34Z

1. Volume relaxation

← Older revision		Revision as of 08:45, 9 September 2024
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	=== Step 2. ===		=== Step 2. ===
	Copy the {{TAG\|CONTCAR}} to {{TAG\|POSCAR}} and relax the structure again. ~~Use~~ {{TAG\|ISTART}} = 1 ~~for this step (~~this keeps that energy cut-off constant, while ~~changing~~ the basis).		Copy the {{TAG\|CONTCAR}} to {{TAG\|POSCAR}} and relax the structure again.
			{{TAG\|ISMEAR}} = 0 ; {{TAG\|SIGMA}} = 0.05 # change smearing to {{TAG\|ISMEAR}} = 1 ; {{TAG\|SIGMA}} = 0.2 for metals
			{{TAG\|ISIF}} = 7 ; {{TAG\|IBRION}} = 2; {{TAG\|NSW}} = 20
			{{TAG\|ISTART}} = 1 # this keeps that energy cut-off constant, while updating the plane wave basis for the new cell volume

	=== Step 3. ===		=== Step 3. ===
	Change the smearing method to the tetrahedron method (i.e. {{TAG\|ISMEAR}}=-5) and perform a single point calculation, i.e. no relaxation of structure. These will give highly accurate energies.		Change the smearing method to the tetrahedron method (i.e. {{TAG\|ISMEAR}}=-5) and perform a single point calculation, i.e. no relaxation of structure. These will give highly accurate energies.
			{{TAG\|ISMEAR}} = -5 # tetrahedron method
			{{TAG\|NSW}} = 0

	=== (Optional) ===		=== (Optional) ===

Csheldon: /* Step 2. */

2024-09-09T08:29:52Z

Step 2.

← Older revision		Revision as of 08:29, 9 September 2024
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	=== Step 2. ===		=== Step 2. ===
	Copy the {{TAG\|CONTCAR}} to {{TAG\|POSCAR}} and relax the structure again.		Copy the {{TAG\|CONTCAR}} to {{TAG\|POSCAR}} and relax the structure again. Use {{TAG\|ISTART}} = 1 for this step (this keeps that energy cut-off constant, while changing the basis).

	=== Step 3. ===		=== Step 3. ===
	Change the smearing method to the tetrahedron method (i.e. {{TAG\|ISMEAR}}=-5) and perform a single point calculation, i.e. no relaxation of structure. These will give highly accurate energies.		Change the smearing method to the tetrahedron method (i.e. {{TAG\|ISMEAR}}=-5) and perform a single point calculation, i.e. no relaxation of structure. These will give highly accurate energies.

Csheldon: /* How to correct */

2024-08-30T09:11:57Z

How to correct

← Older revision		Revision as of 09:11, 30 August 2024
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	= How to correct =		= How to correct =
	The Pulay stress may be corrected in multiple ways. Generally, by calculating the relaxed structure with a larger basis set by increasing the {{TAG\|ENCUT}} until convergence is reached:		The Pulay stress may be corrected in multiple ways. Generally, by calculating the relaxed structure with a larger basis set by increasing the {{TAG\|ENCUT}} until convergence is reached:
	#Set {{TAG\|ENCUT}} to <span>1.3 ×</span> the default cutoff, or {{TAG\|PREC}}=''High'' in VASP.4.4. ~~N.B.~~ this is the lower recommended limit.		#Set {{TAG\|ENCUT}} to <span>1.3 ×</span> the default cutoff, or {{TAG\|PREC}}=''High'' in VASP.4.4. Note: this is the lower recommended limit.
	#Re-run VASP with the default cutoff to obtain the final relaxed positions and cell parameters.		#Re-run VASP with the default cutoff to obtain the final relaxed positions and cell parameters.
	#Further increase the {{TAG\|ENCUT}} and repeat Steps 1 and 2, until the structure no longer changes, i.e. is converged.		#Further increase the {{TAG\|ENCUT}} and repeat Steps 1 and 2, until the structure no longer changes, i.e. is converged.
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	=== Step 1. ===		=== Step 1. ===
	Relax from the starting structure ({{TAG\|ISIF}} = 7; {{TAG\|IBRION}} = 1, 2, or 3) with Gaussian or Methfessel-Paxton smearing ({{TAG\|ISMEAR}} = 0 or 1). ~~N.B.~~ other settings can be used for {{TAG\|ISIF}}, e.g. 3, 6, or 8 but it is not recommended to relax more than one degree of freedom at a time. For instance,		Relax from the starting structure ({{TAG\|ISIF}} = 7; {{TAG\|IBRION}} = 1, 2, or 3) with Gaussian or Methfessel-Paxton smearing ({{TAG\|ISMEAR}} = 0 or 1). Note: other settings can be used for {{TAG\|ISIF}}, e.g. 3, 6, or 8 but it is not recommended to relax more than one degree of freedom at a time. For instance,
	{{TAG\|ISMEAR}} = 0 ; {{TAG\|SIGMA}} = 0.05 # change smearing to {{TAG\|ISMEAR}} = 1 ; {{TAG\|SIGMA}} = 0.2 for metals		{{TAG\|ISMEAR}} = 0 ; {{TAG\|SIGMA}} = 0.05 # change smearing to {{TAG\|ISMEAR}} = 1 ; {{TAG\|SIGMA}} = 0.2 for metals
	{{TAG\|ISIF}} = 7 ; {{TAG\|IBRION}} = 2; {{TAG\|NSW}} = 20		{{TAG\|ISIF}} = 7 ; {{TAG\|IBRION}} = 2; {{TAG\|NSW}} = 20

Csheldon: /* How to correct */

2024-08-19T14:35:32Z

How to correct

← Older revision		Revision as of 14:35, 19 August 2024
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	#Re-run VASP with the default cutoff to obtain the final relaxed positions and cell parameters.		#Re-run VASP with the default cutoff to obtain the final relaxed positions and cell parameters.
	#Further increase the {{TAG\|ENCUT}} and repeat Steps 1 and 2, until the structure no longer changes, i.e. is converged.		#Further increase the {{TAG\|ENCUT}} and repeat Steps 1 and 2, until the structure no longer changes, i.e. is converged.

	{{NB\|mind\|This relaxation is done along all of the lattice vectors. If you want to relax only one and keep the others fixed, e.g. for the vacuum in a surface relaxation, then {{TAG\|LATTICE_CONSTRAINTS}} may be used instead. This can constrain any of the three lattice vectors for orthorhombic cells, or specific rows and columns in the lattice matrix for non-orthorhombic cells.}}		{{NB\|mind\|This relaxation is done along all of the lattice vectors. If you want to relax only one and keep the others fixed, e.g. for the vacuum in a surface relaxation, then {{TAG\|LATTICE_CONSTRAINTS}} may be used instead. This can constrain any of the three lattice vectors for orthorhombic cells, or specific rows and columns in the lattice matrix for non-orthorhombic cells.}}

Csheldon: /* How to correct */

2024-08-19T14:35:26Z

How to correct

← Older revision		Revision as of 14:35, 19 August 2024
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	#Further increase the {{TAG\|ENCUT}} and repeat Steps 1 and 2, until the structure no longer changes, i.e. is converged.		#Further increase the {{TAG\|ENCUT}} and repeat Steps 1 and 2, until the structure no longer changes, i.e. is converged.

	{{NB\|mind\|		{{NB\|mind\|This relaxation is done along all of the lattice vectors. If you want to relax only one and keep the others fixed, e.g. for the vacuum in a surface relaxation, then {{TAG\|LATTICE_CONSTRAINTS}} may be used instead. This can constrain any of the three lattice vectors for orthorhombic cells, or specific rows and columns in the lattice matrix for non-orthorhombic cells.}}
	This relaxation is done along all of the lattice vectors. If you want to relax only one and keep the others fixed, e.g. for the vacuum in a surface relaxation, then {{TAG\|LATTICE_CONSTRAINTS}} may be used instead. This can constrain any of the three lattice vectors for orthorhombic cells, or specific rows and columns in the lattice matrix for non-orthorhombic cells.
	}}

	If volume relaxations are necessary, the following two procedures may be followed:		If volume relaxations are necessary, the following two procedures may be followed:

Csheldon: /* How to correct */

2024-08-19T14:35:08Z

How to correct

← Older revision		Revision as of 14:35, 19 August 2024
Line 10:		Line 10:
	#Re-run VASP with the default cutoff to obtain the final relaxed positions and cell parameters.		#Re-run VASP with the default cutoff to obtain the final relaxed positions and cell parameters.
	#Further increase the {{TAG\|ENCUT}} and repeat Steps 1 and 2, until the structure no longer changes, i.e. is converged.		#Further increase the {{TAG\|ENCUT}} and repeat Steps 1 and 2, until the structure no longer changes, i.e. is converged.

			{{NB\|mind\|
			This relaxation is done along all of the lattice vectors. If you want to relax only one and keep the others fixed, e.g. for the vacuum in a surface relaxation, then {{TAG\|LATTICE_CONSTRAINTS}} may be used instead. This can constrain any of the three lattice vectors for orthorhombic cells, or specific rows and columns in the lattice matrix for non-orthorhombic cells.
			}}

	If volume relaxations are necessary, the following two procedures may be followed:		If volume relaxations are necessary, the following two procedures may be followed:

Csheldon at 12:45, 13 August 2024

2024-08-13T12:45:21Z

← Older revision		Revision as of 12:45, 13 August 2024
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	</li>		</li>
	</ol>		</ol>
	[[Category:PAW ~~method~~]][[Category:Volume relaxation]][[Category:Howto]]		[[Category:PAW]][[Category:Volume relaxation]][[Category:Howto]]

	==References==		==References==

Csheldon: /* Introduction */

2024-08-13T12:44:01Z

Introduction

← Older revision		Revision as of 12:44, 13 August 2024
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	= Introduction =		= Introduction =
	When relaxing the volume of cells, care must be taken. If too small an {{TAG\|ENCUT}} or k-mesh are used, then discontinuities in energy vs. volume curves can be seen, cf. Fig. 1 and 2. This is due to an unconverged basis set being used. This creates unphysical forces, known as [[Pulay ~~Stress~~\|Pulay stress]], that distort the cell structure away from equilibrium, decreasing the volume. This Pulay stress can be neglected in volume-conserving relaxations but becomes important in volume relaxations.		When relaxing the volume of cells, care must be taken. If too small an {{TAG\|ENCUT}} or k-mesh are used, then discontinuities in energy vs. volume curves can be seen, cf. Fig. 1 and 2. This is due to an unconverged basis set being used. This creates unphysical forces, known as [[Pulay stress\|Pulay stress]], that distort the cell structure away from equilibrium, decreasing the volume. This Pulay stress can be neglected in volume-conserving relaxations but becomes important in volume relaxations.

	[[File:ENCUT_comp.png\|400px\|thumb\|Figure 1. Total energy vs. lattice parameter for converged and unconverged plane wave energy cutoffs. Diamond in a primitive cell - 2x2x2 k-point mesh.]]		[[File:ENCUT_comp.png\|400px\|thumb\|Figure 1. Total energy vs. lattice parameter for converged and unconverged plane wave energy cutoffs. Diamond in a primitive cell - 2x2x2 k-point mesh.]]