3. Example: Isolated Systems¶
This example shows how to use pw.x to simulate isolated charge systems in vacuum or immersed in a continuum dielectric constant. Note that this can be thought of as an alternative to Environ’s own periodic correction as seen in Example 1. Since this setting resides in the pw input file, both input files should be checked for consistence. In the first example we removed all periodicity from the system with a correction that results in a fairly good approximation.
&ELECTROSTATIC
pbc_correction = 'parabolic'
pbc_dim = 0
In this example, we demonstrate a system without periodicity, thus removing the parameters stated before and keeping to the defaults. We also demonstrate an alternative to the pbc correction, which is the martyna-tuckerman [1] correction. This correction is seen as a more accurate method, at the cost of a larger required cellsize. For reliable results, a cell length of twice the molecule length is recommended, in any direction. This can be added into the pw input file, under the SYSTEM keyword.
&SYSTEM
assume_isolated = 'martyna-tuckerman'
The tolerance is different from previous examples. This value is chosen to facilitate the self-consistent process and will directly affect the speed and accuracy of the convergence. One should set this according to the type of simulation being run, these values in the examples serving as guidelines for that decision.
Clearly for larger systems, the parabolic correction is a more feasible choice for imposing isolation. This is a real-space quadratic correction of the electrostatic potential [2], which has been shown to provide energy accuracy improvements of almost 1 order of magnitude in comparison with the alternative PCC and GCC schemes for large cell sizes. This correction scheme implemented in Environ is limited to 0D and 2D systems (which account for the majority of systems that Environ typically works with).
| [1] |
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| [2] | I. Dabo et al. Phys. Rev. B 77, 115139 (2008) |