Nudged elastic bands: Difference between revisions
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The nudged elastic band (NEB) method | The nudged elastic band (NEB) method<ref name="jons95"/><ref name="jons98"/> is a technique used to calculate energy barriers in VASP. When employing this method, the SPRING parameter is set to a negative value (e.g., SPRING= -5), which is the recommended setting. This negative value introduces additional tangential springs to maintain equidistance among images during the relaxation process. It is important not to use excessively large values for SPRING, as it can hinder convergence. The default value generally provides reliable results. | ||
One challenge with the NEB method arises from its non-linear constraint, where movements are restricted to the hyper-plane perpendicular to the current tangent. This characteristic can lead to convergence issues with the Conjugate Gradient (CG) algorithm. In such cases, it is advisable to use alternative algorithms like the RMM-DIIS algorithm (IBRION=1) or the quick-min algorithm (IBRION=3). Additionally, the equidistant images tend to deviate from this constraint in the initial steps. To address this, applying a low dimensionality parameter (IBRION=1, NFREE=2) in the initial steps or using steepest descent minimization without line optimization (IBRION=3, SMASS=2) can help pre-converge the images. | One challenge with the NEB method arises from its non-linear constraint, where movements are restricted to the hyper-plane perpendicular to the current tangent. This characteristic can lead to convergence issues with the Conjugate Gradient (CG) algorithm. In such cases, it is advisable to use alternative algorithms like the RMM-DIIS algorithm (IBRION=1) or the quick-min algorithm (IBRION=3). Additionally, the equidistant images tend to deviate from this constraint in the initial steps. To address this, applying a low dimensionality parameter (IBRION=1, NFREE=2) in the initial steps or using steepest descent minimization without line optimization (IBRION=3, SMASS=2) can help pre-converge the images. | ||
Revision as of 11:10, 18 October 2023
The nudged elastic band (NEB) method[1][2] is a technique used to calculate energy barriers in VASP. When employing this method, the SPRING parameter is set to a negative value (e.g., SPRING= -5), which is the recommended setting. This negative value introduces additional tangential springs to maintain equidistance among images during the relaxation process. It is important not to use excessively large values for SPRING, as it can hinder convergence. The default value generally provides reliable results.
One challenge with the NEB method arises from its non-linear constraint, where movements are restricted to the hyper-plane perpendicular to the current tangent. This characteristic can lead to convergence issues with the Conjugate Gradient (CG) algorithm. In such cases, it is advisable to use alternative algorithms like the RMM-DIIS algorithm (IBRION=1) or the quick-min algorithm (IBRION=3). Additionally, the equidistant images tend to deviate from this constraint in the initial steps. To address this, applying a low dimensionality parameter (IBRION=1, NFREE=2) in the initial steps or using steepest descent minimization without line optimization (IBRION=3, SMASS=2) can help pre-converge the images.
If all degrees of freedom are allowed to relax (e.g., in isolated molecules or surfaces), it is crucial to ensure that the sum of all positions remains consistent across all cells. Failing to do so introduces artificial forces, causing the images to drift apart. While this doesn't affect the VASP calculations, it can complicate result visualization. Often, an initial linearly interpolated starting guess is appropriate, which can be achieved using a script called interpolatePOS. This script also offers the option to remove center-of-mass motion.
It is highly recommended to minimize the number of images used to an absolute minimum. Convergence to the ground state is faster with fewer images. Starting with a single image between the two endpoints and increasing the number of images after the initial run has converged is often a prudent approach.
References
- ↑ G. Mills, H. Jonsson and G. K. Schenter, Surface Science, 324, 305 (1995).
- ↑ H. Jonsson, G. Mills and K. W. Jacobsen, Nudged Elastic Band Method for Finding Minimum Energy Paths of Transitions, in Classical and Quantum Dynamics in Condensed Phase Simulations, ed. B. J. Berne, G. Ciccotti and D. F. Coker (World Scientific, 1998).