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

```Dear Nick Papior,
misunderstood your response. As shown in the comments section below (red font) :
1. Please confirm whether the comments in red font below are correct. This will
help me a lot.
%block Geometry.Hartree
plane 1. eV # The lifting potential on the geometry
delta
1.0 1.0 1.0 Ang # An intersection point, in the plane
1.0 0.5 0.2 # The normal vector to the plane
%endblock Geometry.Hartree
(comment) : In the above example, (1.0 1.0 1.0) is the starting point of the
normal vector and (1.0 0.5 0.2) is the end point of the normal vector. Since it
is an infinite plane, the length of the normal vector can be arbitrary.
%block Geometry.Hartree
square 1. eV # The lifting potential on the geometry
delta
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
(comment) : In the above example, (1.0 1.0 1.0) is the starting point of the
first and second spanning vectors, and (2.0 0.5 0.2) (0.0 2.5 0.2) is the end
point of the first and second spanning vectors, respectively. The first and
second spanning vectors form a parallelogram. The lengths of the first and
second spanning vectors determine the area of the parallelogram, which is the
size of the square.
%block Geometry.Hartree
box 1. eV # The lifting potential on the geometry
delta
1.0 1.0 1.0 Ang # Origo of the box
2.0 0.5 0.2 Ang # The first spanning vector
0.0 2.5 0.2 Ang # The second spanning vector
0.0 0.5 3.2 Ang # The third spanning vector
%endblock Geometry.Hartree
(comment) : In the example above (1.0 1.0 1.0) is the starting point of the
first, second, and third spanning vectors. (2.0 0.5 0.2) (0.0 2.5 0.2) (0.0 0.5
3.2) are the end points of the first, second and third spanning vectors
respectively. The three spanning vectors form a parallelepiped. The lengths of
the three spanning vectors determine the volume of the parallelepiped, which is
the volume of the box.
2. As for your reply "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. ", I do not understand how long the vacuum layer between the structure
and the gate is appropriate.
Thank you very much!
Wei```
```

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xiaowei951...@163.com
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On 7/25/2022 04:00，Nick Papior<nickpap...@gmail.com> wrote：
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 method of adding gate
voltage to a system in this article and try to learn 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

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xiaowei951...@163.com
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--
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/)
```