# Re: [SIESTA-L] Question about Gate in SIESTA

```Hi,

On Fri, 1 Jul 2022, 22:00 肖威, <xiaowei951...@163.com> wrote:```
```
> Dear SIESTA developers and users,
>
> I recently read Papior et al. 's article on Phys. Chem. Chem. Phys.
> entitled "Manipulating the voltage drop in graphene nanojunctions using a
> gate potential (DOI: 10.1039/C5CP04613K)". I am very interested in the
> how to use it. However, I have encountered some difficulties in the
> process, and I'm really looking forward to some help.
>
> 1. When I use the square (Bounded plane) option in Gate, I need to set
> the starting point of the square and two spanning vectors. As shown below
> (copied from the SIESTA 4.1-b4 manual).
>
> %block Geometry.Hartree
>
> square 1. eV          # The lifting potential on the geometry
>
> gauss 1. 2. Ang      # the std. and the cut-off length
>
> 1.0  1.0  1.0  Ang  # The starting point of the square
>
> 2.0  0.5  0.2  Ang  # The first spanning vector
>
> 0.0  2.5  0.2  Ang  # The second spanning vector
>
> %endblock Geometry.Hartree
>
> But we all know that two points define a vector, so do the coordinates
> that define the spanning vector in the example above represent the end
> point of the vector? If so, what is the starting point of this spanning
> vector? Is the spanning vector starting at the origin (0  0  0) or at the
> starting point of the square (1.0  1.0  1.0) defined in the example above?
>
> I have the same question about plane (Infinite plane, a vector) and Box
> (three vectors) in Gate.
>

There are 3 points, (1,2,3) the vectors go from 1-2 and 1-3.
In the square geometry the vectors form a bounded surface, in the infinite
plane the length of them doesn't matter as they will be considered
infinite.

> 2. Whether a shorter vacuum layer in the direction of adding Gate makes
> self-consistency difficult to converge. How to determine the appropriate
> length of vacuum layer?
>

The most simple thing is (if you don't have periodicity along the field
generated by the gate) to add a slab dipole correction, and add vacuum
corresponding to 1.5 times the distance between your structure and the
gate.

Note however that the hartree gate merely changes the electrostatic
potential in the defined region, i.e. it is not a boundary condition in the
generic sense.

> I'm really looking forward to some help.
>
> Thank you very much!
>
> Wei
>
> 肖威
> xiaowei951...@163.com
>
> <https://maas.mail.163.com/dashi-web-extend/html/proSignature.html?ftlId=1&name=%E8%82%96%E5%A8%81&uid=xiaowei951020%40163.com&iconUrl=https%3A%2F%2Fmail-online.nosdn.127.net%2Fqiyelogo%2FdefaultAvatar.png&items=%5B%22xiaowei951020%40163.com%22%5D>
>
> --
> SIESTA is supported by the Spanish Research Agency (AEI) and by the
> European H2020 MaX Centre of Excellence (http://www.max-centre.eu/)
>
```
```--
SIESTA is supported by the Spanish Research Agency (AEI) and by the European
H2020 MaX Centre of Excellence (http://www.max-centre.eu/)
```