[QE-users] Defining C diamond structure cp.x

[Stefano Baroni]

Thank you, Michal. When you are done, please share your converter with the community. You may eve want to consider distributing it with QE. Thanks and best wishes to all. SB

___
Stefano Baroni, Trieste -- http://stefano.baroni.me

> On 28 Feb 2022, at 20:45, Husak Michal <Michal.Husak at vscht.cz> wrote:
> 
> The most save and correct way is not to use ibraw, but space group number ...
> I will generate correct diamond for you toomorow (I work
> with my students on CIF->QE convertor with GUI ...
> But diamond is so simple, it can be done by hand ....
> You need just the SG number (i do not remember Fd3m number)
> and only 1 atom in 0,0,0 ...
> 
> Michal Husak
> UCT Prague
> ________________________________
> From: users <users-bounces at lists.quantum-espresso.org> on behalf of Juniper Savchick <curly363 at gmail.com>
> Sent: Monday, February 28, 2022 8:31:49 PM
> To: users at lists.quantum-espresso.org
> Subject: [QE-users] Defining C diamond structure cp.x
> 
> Hello,
> 
> I am a graduate assistant at Virginia Commonwealth University and I am using Quantum Espresso to perform CP molecular dynamics calculations. I would like to run a simple set up of a single Carbon Diamond crystal structure. My ultimate research goal is to generate equations of state for warm dense matter, starting with Carbon in a diamond structure. It is my understanding that there are multiple ways to input this system into Quantum Espresso but I am not getting the same results with each method. I want to make sure I have an intimate understanding of the cp.x executable. This is why I want to calculate the same system using different inputs. (I am using QE 7.0) I have tried many different input methods with the main differences being the number of atoms that are explicitly defined. (2,8,& 18)
> 
> It is my understanding that if you use input parameter ibrav=2 where 2 atoms are explicitly defined under "ATOMIC_POSITIONS", this is the carbon diamond primitive cell. Based on QE related literature, cp.x will automatically fill in the remaining atoms to complete the carbon diamond crystal structure. I am skeptical that cp.x is actually doing this. Most of the QE related literature pertained to pw.x so perhaps each executable defines the crystal structure differently? I have looked into documentation for both cp.x and pw.x to no avail. I would appreciate anyone who can help clarify this input for me.
> 
> If you use input parameter ibrav=0 then you should be able to explicitly define all 18 atoms that comprise the conventional carbon diamond lattice structure. However, if I try to use ibrav=0 and explicitly define 18 atomic positions I get infinite values as outputs. My next attempt was to use ibrav=0 and explicitly define 8 atomic positions. The 8 atom run gave reasonable values but the pressure was orders of magnitude smaller than the 2 atom run. This is concerning to me and it has become clear that I am misunderstanding the cp.x input. I have scoured QE forums and documentation to no avail.
> 
> At this point it would seem the most accurate method is the 8 atom run since I can use these positions to generate a supercell that has the periodic carbon diamond structure, albeit with vacancies in some of the corners and sides. I assume if I make the supercell large enough the errors due to imperfections will be negligible. Also, no real structure is perfectly periodic so we should still be able to use the 8 atom unit cell to construct at usable supercell for modeling purposes. Does this assumption sound reasonable?
> 
> To summarize by questions:
> What is the most accurate way to input the carbon diamond crystal structure in cp.x?
> Why does ibrav=0 with 18 atoms explicitly defined not work?
> Is my understanding of how to input the primitive cell in cp.x correct?
> Is my assumption regarding the "8 atom generated supercell" reasonable?
> 
> I understand this is quite a bit to ask but I am at a loss for what to do next. I would appreciate any help that can be offered. I have provided example input files for each type of input described. (see google drive link)
> 
> Sincerely,
> Juniper Savchick
> Graduate Assistant
> Virginia Commonwealth University
> BS Physics, minor Mathematics
> Pursuing a PhD in Mechanical and Nuclear Engineering
> 
> [https://drive-thirdparty.googleusercontent.com/16/type/application/vnd.google-apps.folder+shared] QE inputs<https://drive.google.com/drive/folders/1BLY8nVULkYzAWleKz-n6nwjFaKHc_U4m>
> 
> 
> _______________________________________________
> Quantum ESPRESSO is supported by MaX (www.max-centre.eu)
> users mailing list users at lists.quantum-espresso.org
> https://lists.quantum-espresso.org/mailman/listinfo/users