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

[Husak Michal]

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>