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

[Juniper Savchick]

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

 QE inputs
<https://drive.google.com/drive/folders/1BLY8nVULkYzAWleKz-n6nwjFaKHc_U4m>
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