Energy per unit cell in NWChem differs for different supercells

[Dee Mee]

I try to compute the unit cell energy of the Ti2CO2 MXene with NWchem.

Computing the 1x1 supercell with 4x4x1 K-points grid I got -1875 Ha.

Computing the 2x2 supercell with 2x2x1 K-points grid I got -7502 Ha, that means -1875.5 Ha per unit cell.

I understand, that for useful results convergence should be reached with K-points and the basis set. But here I just want to understand the source of the 0.5 Ha discrepancy. Previously I thought, that NxM supercell is equivalent to NxM K-points grid. Apparently it is not true. What is the source of such a significant difference?

Inputs for both cases are attached.

I also have asked this question in StackExchange Materials Modelling group here. But for a while there are no decisive answers.

[Bylaska, Eric J]

 

It looks like you’re trying to use the Gaussian DFT code in NWChem.  This code does not contain periodic boundary conditions.  Only the plane-wave band code in the nwpw module can do periodic dft calculations with k-points.

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[Dee Mee]

Thank you, Eric! You helped a lot!

Indeed, NWchem seems to supports k-points only in its plane-wave module. I should have noticed that the monkhorst-pack option is in the nwpw section and it is suspicious.

But I still think, that periodic boundary conditions are supported in the Gaussian DFT code. At least a see difference in energy between system crystal and without this section. Please, correct me if I am wrong.

[Edoardo Aprà]

No, periodic boundary conditions are not supported in the Gaussian DFT code.

I have opened a github issue about the code not stopping when these conditions appear in the input file

https://github.com/nwchemgit/nwchem/issues/322

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Is it just about 3D PBC (system crystal)? Or all periodic systems are not implemented for Gaussian DFT? The documentation here tells, that 2D (surface) and 1D (polymer) symmetries are "not currently available with NWPW module".  But where are they available?

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[Dee Mee]

Is it just about 3D PBC (system crystal)? Or all periodic systems are not implemented for Gaussian DFT? The documentation here tells, that 2D (surface) and 1D (polymer) symmetries are "not currently available with NWPW module".  But where are they available?

[Eric Bylaska]

The plane wave code automatically places 2d and 1d periodic in a padded 3D cell

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From: nwchem...@googlegroups.com <nwchem...@googlegroups.com> on behalf of Dee Mee <maz...@gmail.com>
Sent: Friday, February 12, 2021 7:23:59 PM
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Subject: Re: [nwchem-forum] Energy per unit cell in NWChem differs for different supercells

 

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[Dee Mee]

So it means, that for Gaussian basis there are neither 3D, nor 2D, nor 1D periodic boundaries. Right?

[Edoardo Aprà]

https://groups.google.com/g/nwchem-forum/c/CYQeUO-zZv0/m/eoSiGn7fBAAJ