Intrinsic-reaction-coordinate calculations: Difference between revisions
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The potential energy profiles along intrinsic reaction coordinate (IRC) can be computed via | The potential energy profiles along the '''intrinsic reaction coordinate''' (IRC) can be computed via the method of Hratchian and Schlegel{{Cite|hratchian:jpc:2002}}. The algorithm starts from the transition state and propagates the system via the damped-velocity-Verlet algorithm. The damping is realized via rescaling the velocity vector to a constant value (<math>v_0</math>) after each propagation step. At the same time, the time step is adaptively changed so as to ensure that the trajectory generated by the algorithm does not differ from true IRC by more than the predefined tolerance factor <math>\Delta_0</math>. As an input, the structure of a well-relaxed transition state and the direction of the unstable vibration mode must be provided. For that purpose, a {{FILE|CONTCAR}} file from an [[Improved Dimer Method|improved-dimer-method]] calculation converged with a tight relaxation criterion (e.g., {{TAG|EDIFFG }}=-0.005) can be used. To obtain a complete energy profile along the IRC connecting two stable states, two independent calculations with positive ({{TAG|IRC_DIRECTION }}=1) and negative ({{TAG|IRC_DIRECTION }}=-1) initial displacement along the direction of the unstable mode must be performed. | ||
The following parameters can be modified to affect the performance of the method: | The following parameters can be modified to affect the performance of the method: | ||
*{{TAG|IRC_DIRECTION }} direction of the initial displacement (-1|1 – negative|positive) | *{{TAG|IRC_DIRECTION }} direction of the initial displacement (-1|1 – negative|positive) | ||
*{{TAG|IRC_STOP}} the number of steps the energy must monotonously increase before the algorithm terminates. In order to avoid a premature | *{{TAG|IRC_STOP}} the number of steps the energy must monotonously increase before the algorithm terminates. In order to avoid a premature termination, especially close to transition states., e.g., due to numerical noise, {{TAG|IRC_STOP}} should always be greater than 1. | ||
*{{TAG|IRC_DELTA0}} the tolerance factor <math>\Delta_0</math> in Å – the smaller value, the closer the computed trajectory follows the true IRC (but the more DFT steps | *{{TAG|IRC_DELTA0}} the tolerance factor <math>\Delta_0</math> in Å – the smaller the value, the closer the computed trajectory follows the true IRC (but the more DFT steps are required) | ||
*{{TAG|IRC_MINSTEP}} specifies the lower limit for the time step in fs | *{{TAG|IRC_MINSTEP}} specifies the lower limit for the time step in fs | ||
*{{TAG|IRC_MAXSTEP}} specifies the upper limit for the time step in fs | *{{TAG|IRC_MAXSTEP}} specifies the upper limit for the time step in fs | ||
*{{TAG|IRC_VNORM0}} the value of <math>v_0</math> in Å/fs | *{{TAG|IRC_VNORM0}} the value of <math>v_0</math> in Å/fs | ||
{{NB|mind|2=This method is presently available only for fixed cell shape (i.e., {{TAG|ISIF}} = 2) simulations.}} | {{NB|mind|2=This method is presently available only for fixed cell shape (i.e., {{TAG|ISIF}} = 2) simulations.}} | ||
{{NB|mind|2=The calculation must be initialized from a very well relaxed transition state ({{TAG|EDIFFG}} = -0.005 or less in absolute value).}} | {{NB|mind|2=The calculation must be initialized from a very well-relaxed transition state ({{TAG|EDIFFG}} = -0.005 or less in absolute value).}} | ||
== Practical example == | == Practical example == | ||
Revision as of 05:30, 12 November 2023
The potential energy profiles along the intrinsic reaction coordinate (IRC) can be computed via the method of Hratchian and Schlegel[1]. The algorithm starts from the transition state and propagates the system via the damped-velocity-Verlet algorithm. The damping is realized via rescaling the velocity vector to a constant value () after each propagation step. At the same time, the time step is adaptively changed so as to ensure that the trajectory generated by the algorithm does not differ from true IRC by more than the predefined tolerance factor . As an input, the structure of a well-relaxed transition state and the direction of the unstable vibration mode must be provided. For that purpose, a CONTCAR file from an improved-dimer-method calculation converged with a tight relaxation criterion (e.g., EDIFFG =-0.005) can be used. To obtain a complete energy profile along the IRC connecting two stable states, two independent calculations with positive (IRC_DIRECTION =1) and negative (IRC_DIRECTION =-1) initial displacement along the direction of the unstable mode must be performed.
The following parameters can be modified to affect the performance of the method:
- IRC_DIRECTION direction of the initial displacement (-1|1 – negative|positive)
- IRC_STOP the number of steps the energy must monotonously increase before the algorithm terminates. In order to avoid a premature termination, especially close to transition states., e.g., due to numerical noise, IRC_STOP should always be greater than 1.
- IRC_DELTA0 the tolerance factor in Å – the smaller the value, the closer the computed trajectory follows the true IRC (but the more DFT steps are required)
- IRC_MINSTEP specifies the lower limit for the time step in fs
- IRC_MAXSTEP specifies the upper limit for the time step in fs
- IRC_VNORM0 the value of in Å/fs
| Mind: This method is presently available only for fixed cell shape (i.e., ISIF = 2) simulations. |
| Mind: The calculation must be initialized from a very well-relaxed transition state (EDIFFG = -0.005 or less in absolute value). |