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

[Tone Kokalj]

Dear Juniper,

I don't know about cp.x specifics (I'm using pw.x) and I also do not 
understand your email.
But otherwise diamond structure is really simple to input. It should be 
something like:

&SYSTEM
   ibrav = 2
   ntyp = 1
   nat = 2
/

ATOMIC_POSITIONS (crystal)
C  -0.125 -0.125 -0.125
C   0.125  0.125  0.125

The conventional cell (cubic) is 4x bigger, hence there are 8 
atoms/conv-cell.

Regards, Tone

On 2022-02-28 20:31, Juniper Savchick wrote:
> 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 [1]
> 
> 
> 
> Links:
> ------
> [1] 
> https://drive.google.com/drive/folders/1BLY8nVULkYzAWleKz-n6nwjFaKHc_U4m
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