# [Pw_forum] Lattice Constant of ESM Slab

Minoru Otani minoru.otani at aist.go.jp
Sat Jul 29 00:20:03 CEST 2017

```Dear Robert,

There are many ways to define a lattice in input files. So I give you a general expression of the unit cell in matrix form:
2a    0           0
0    2a          0
0     0    10+4a+10

Please remember that, as shown in the diagram on (http://sugino.issp.u-tokyo.ac.jp/esm/index.php?SIESTA%2BESM%20Manual), L is the length of the unit cell along z-axis. Thus L=10 + 4a + 10 is correct.

Best regards,
Minoru
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Minoru Otani
National Institute of Advanced Industrial Science and Technology
Research Centre for Computational Design of Advanced Functional Materials
email : minoru.otani at aist.go.jp
tel : +81-29-861-5202
fax : +81-29-861-3171
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> On Jul 29, 2017, at 0:20, Dr. Robert Molt Jr. <r.molt.chemical.physics at gmail.com> wrote:
>
> Good afternoon,
>
> I do not understand how the lattice constant is defined in an ESM
> calculation of a slab.
>
> Let's say I have a 2x2x4 slab, and wish to simulate a vacuum on either
> side of it. Let us say the unit cells are simple cubic, such that the
> height is "a". For a 10 Angstrom vacuum, I assume L/2=10, pursuant to
> the diagram on
> (http://sugino.issp.u-tokyo.ac.jp/esm/index.php?SIESTA%2BESM%20Manual).
> If I do
>
> assume_isolated=esm
>
> esm_bc=bc1
>
> Is the lattice constant for this situation defined as
>
> celldm(1)= L/2 + 4a + L/2
>
> ?
>
> Dr. Robert Molt
>
> Indiana University Perdue University
>
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```