Calculation of dimers - Revision history

Schlipf at 06:03, 20 June 2022

2022-06-20T06:03:33Z

← Older revision		Revision as of 06:03, 20 June 2022
Line 21:		Line 21:
	For this {{TAG\|POSCAR}} file the starting distance is 2.56 <math>\AA</math>, in each step the distance is reduced by 0.04 <math>\AA</math>, leading to a final distance of 2.20 <math>\AA</math>. The obtained energies can be fitted to a Morse potential.		For this {{TAG\|POSCAR}} file the starting distance is 2.56 <math>\AA</math>, in each step the distance is reduced by 0.04 <math>\AA</math>, leading to a final distance of 2.20 <math>\AA</math>. The obtained energies can be fitted to a Morse potential.

	Mind: In some rare cases like C<sub>2</sub>, the calculation of the dimer turns out to be problematic. For this case the LUMO (lowest unoccupied molecular orbital) and the HOMO (highest occupied molecular orbital) cross at a certain distance, and are actually degenerate, if the total energy is used as a variational quantity (i.e. <math>sigma \to 0</math>). Within the finite temperature LDA these difficulties are avoided, but interpreting the results is not easy because of the finite entropy (for C<sub>2</sub> see Ref. {{cite\|pederson:prb:1991}}).		Mind: In some rare cases like C<sub>2</sub>, the calculation of the dimer turns out to be problematic. For this case the LUMO (lowest unoccupied molecular orbital) and the HOMO (highest occupied molecular orbital) cross at a certain distance, and are actually degenerate, if the total energy is used as a variational quantity (i.e. <math>\sigma \to 0</math>). Within the finite temperature LDA these difficulties are avoided, but interpreting the results is not easy because of the finite entropy (for C<sub>2</sub> see Ref. {{cite\|pederson:prb:1991}}).

	== References ==		== References ==

Karsai at 13:37, 25 June 2019

2019-06-25T13:37:01Z

← Older revision		Revision as of 13:37, 25 June 2019
Line 23:		Line 23:
	Mind: In some rare cases like C<sub>2</sub>, the calculation of the dimer turns out to be problematic. For this case the LUMO (lowest unoccupied molecular orbital) and the HOMO (highest occupied molecular orbital) cross at a certain distance, and are actually degenerate, if the total energy is used as a variational quantity (i.e. <math>sigma \to 0</math>). Within the finite temperature LDA these difficulties are avoided, but interpreting the results is not easy because of the finite entropy (for C<sub>2</sub> see Ref. {{cite\|pederson:prb:1991}}).		Mind: In some rare cases like C<sub>2</sub>, the calculation of the dimer turns out to be problematic. For this case the LUMO (lowest unoccupied molecular orbital) and the HOMO (highest occupied molecular orbital) cross at a certain distance, and are actually degenerate, if the total energy is used as a variational quantity (i.e. <math>sigma \to 0</math>). Within the finite temperature LDA these difficulties are avoided, but interpreting the results is not easy because of the finite entropy (for C<sub>2</sub> see Ref. {{cite\|pederson:prb:1991}}).

			== References ==
			<references/>

			<noinclude>
	----		----
	[[Category:Atoms and Molecules]][[Category:Howto]]		[[Category:Atoms and Molecules]][[Category:Howto]]

Karsai at 13:36, 25 June 2019

2019-06-25T13:36:00Z

← Older revision		Revision as of 13:36, 25 June 2019
Line 21:		Line 21:
	For this {{TAG\|POSCAR}} file the starting distance is 2.56 <math>\AA</math>, in each step the distance is reduced by 0.04 <math>\AA</math>, leading to a final distance of 2.20 <math>\AA</math>. The obtained energies can be fitted to a Morse potential.		For this {{TAG\|POSCAR}} file the starting distance is 2.56 <math>\AA</math>, in each step the distance is reduced by 0.04 <math>\AA</math>, leading to a final distance of 2.20 <math>\AA</math>. The obtained energies can be fitted to a Morse potential.

	Mind: In some rare cases like C<sub>2</sub>, the calculation of the dimer turns out to be problematic. For this case the LUMO (lowest unoccupied molecular orbital) and the HOMO (highest occupied molecular orbital) cross at a certain distance, and are actually degenerate, if the total energy is used as a variational quantity (i.e. <math>sigma \to 0</math>). Within the finite temperature LDA these difficulties are avoided, but interpreting the results is not easy because of the finite entropy (for C~~$ _2$~~ see Ref. ~~[54]~~).		Mind: In some rare cases like C<sub>2</sub>, the calculation of the dimer turns out to be problematic. For this case the LUMO (lowest unoccupied molecular orbital) and the HOMO (highest occupied molecular orbital) cross at a certain distance, and are actually degenerate, if the total energy is used as a variational quantity (i.e. <math>sigma \to 0</math>). Within the finite temperature LDA these difficulties are avoided, but interpreting the results is not easy because of the finite entropy (for C<sub>2</sub> see Ref. {{cite\|pederson:prb:1991}}).

	----		----
	[[Category:Atoms and Molecules]][[Category:Howto]]		[[Category:Atoms and Molecules]][[Category:Howto]]

Karsai at 13:34, 25 June 2019

2019-06-25T13:34:09Z

← Older revision		Revision as of 13:34, 25 June 2019
Line 1:		Line 1:
	Reproducing accurate dimer distances is an important difficult benchmark for a potential. If a potential works accurately for dimers and bulk calculations, one can be quite confident that the potential possesses excellent transferability. For the simulation of the dimers, one can use the <math>\Gamma</math> point and displace the second atom along the diagonal direction. Generally bonding length and vibrational frequency have to be compared with accurate reference data. It is recommended to perform these calculations using the constant velocity molecular dynamic mode (i.e. {{TAG\|IBRION}}=2, {{TAG\|SMASS}}=-2). This mode speeds up the calculation because the wave functions are extrapolated and predicted using information from previous steps. The {{TAG\|INCAR}} file must contain additional lines to perform the constant velocity MD:		Reproducing accurate dimer distances is an important difficult benchmark for a potential. If a potential works accurately for dimers and bulk calculations, one can be quite confident that the potential possesses excellent transferability. For the simulation of the dimers, one can use the <math>\Gamma</math> point and displace the second atom along the diagonal direction. Generally bonding length and vibrational frequency have to be compared with accurate reference data. It is recommended to perform these calculations using the constant velocity molecular dynamic mode (i.e. {{TAG\|IBRION}}=2, {{TAG\|SMASS}}=-2). This mode speeds up the calculation because the wave functions are extrapolated and predicted using information from previous steps. The {{TAG\|INCAR}} file must contain additional lines to perform the constant velocity MD:
	#ionic relaxation		#ionic relaxation
	{{~~TAG~~\|NSW}} = 10 #number of steps for IOM		{{TAGBL\|NSW}} = 10 #number of steps for IOM
	{{~~TAG~~\|SMASS}} = -2 #constant velocity MD		{{TAGBL\|SMASS}} = -2 #constant velocity MD
	{{~~TAG~~\|POTIM}} = 1 #time-step for ionic-motion		{{TAGBL\|POTIM}} = 1 #time-step for ionic-motion

			In addition to the positions the {{TAG\|POSCAR}} file must also contain velocities:
			dimer
			1
			10.00000 .00000 .00000
			.00000 10.00000 .00000
			.00000 .00000 10.00000
			2
			cart
			0 0 0
			1.47802 1.47802 1.47802
			cart
			0 0 0
			-.02309 -.02309 -.02309

			For this {{TAG\|POSCAR}} file the starting distance is 2.56 <math>\AA</math>, in each step the distance is reduced by 0.04 <math>\AA</math>, leading to a final distance of 2.20 <math>\AA</math>. The obtained energies can be fitted to a Morse potential.

			Mind: In some rare cases like C<sub>2</sub>, the calculation of the dimer turns out to be problematic. For this case the LUMO (lowest unoccupied molecular orbital) and the HOMO (highest occupied molecular orbital) cross at a certain distance, and are actually degenerate, if the total energy is used as a variational quantity (i.e. <math>sigma \to 0</math>). Within the finite temperature LDA these difficulties are avoided, but interpreting the results is not easy because of the finite entropy (for C$ _2$ see Ref. [54]).

	----		----
	[[Category:Atoms and Molecules]][[Category:Howto]]		[[Category:Atoms and Molecules]][[Category:Howto]]

Karsai: Created page with "Reproducing accurate dimer distances is an important difficult benchmark for a potential. If a potential works accurately for dimers and bulk calculations, one can be quite co..."

2019-06-25T13:31:00Z

Created page with "Reproducing accurate dimer distances is an important difficult benchmark for a potential. If a potential works accurately for dimers and bulk calculations, one can be quite co..."

New page

Reproducing accurate dimer distances is an important difficult benchmark for a potential. If a potential works accurately for dimers and bulk calculations, one can be quite confident that the potential possesses excellent transferability. For the simulation of the dimers, one can use the <math>\Gamma</math> point and displace the second atom along the diagonal direction. Generally bonding length and vibrational frequency have to be compared with accurate reference data. It is recommended to perform these calculations using the constant velocity molecular dynamic mode (i.e. {{TAG|IBRION}}=2, {{TAG|SMASS}}=-2). This mode speeds up the calculation because the wave functions are extrapolated and predicted using information from previous steps. The {{TAG|INCAR}} file must contain additional lines to perform the constant velocity MD:
#ionic relaxation
{{TAG|NSW}} = 10 #number of steps for IOM
{{TAG|SMASS}} = -2 #constant velocity MD
{{TAG|POTIM}} = 1 #time-step for ionic-motion

----
[[Category:Atoms and Molecules]][[Category:Howto]]