# [Pw_forum] Lattice Constant of ESM Slab

Dr. Robert Molt Jr. r.molt.chemical.physics at gmail.com
Fri Jul 28 22:01:59 CEST 2017

```Addendum:

Below, you noted that periodicity only exists along z-hat in an ESM
calculation. Would a corollary therein be that one needs to "cap"
boundary atoms along the xz and yz planes with hydrogens (or the like)
to fulfill the octets that would otherwise be satisfied by PBCs in those
directions?

On 7/28/2017 12:23 PM, nazari at iasbs.ac.ir wrote:
>   celldm(1)=10+4a
>
>   this is correct. Remember put the 2*2*4 unit cell in the midl of cube
>   with dimension 10+4a. (......5.....aaaa(4a)aaaaa.......5).
>
>
>   regards
>    Fariba
>   IASBS
>
>
>
>> To be explicit, is this correct:
>>
>> celldm(1)=10+4a
>>
>> if it's a 2x2x4 slab, z height is 4a? Or is it
>>
>> celldm(1)=10+2a
>>
>> because the program requires/assumes that there is symmetry in the xy
>> plane for z=0, and has already factored that in?
>>
>> On 7/28/2017 11:53 AM, nazari at iasbs.ac.ir wrote:
>>> Hi,
>>>
>>>    in slab model periodicity is restricted in one dimension, Suppose Z.
>>>
>>>    in your example you should fit the cube with a dimension in the cube
>>> with
>>>
>>>    a+10 dimension.(-...10/2......aaaaaaaaa.....10/2......)
>>>
>>>
>>>    I do hope this help
>>>
>>> regards
>>> Fariba
>>> IASBS
>>>
>>>
>>>
>>>
>>>> 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
>>>>
>>>> _______________________________________________
>>>> Pw_forum mailing list
>>>> Pw_forum at pwscf.org
>>>> http://pwscf.org/mailman/listinfo/pw_forum
>>>>
>>>> --
>>>> This message has been scanned for viruses and
>>>> dangerous content by MailScanner, and is
>>>> believed to be clean.
>>>>
>>>>
>>>
>>
>> --
>> This message has been scanned for viruses and
>> dangerous content by MailScanner, and is
>> believed to be clean.
>>
>>
>
>

```