Binding energy of graphene bilayer: CHARMM vs. DFT-D3

[Robert Yang]

Dear all,

I am calculating the binding energy of graphene bilayer of the size 3nmX3nm (2X416 atoms). It turns out that the result from calculations with classical force field is quite different from that of DFTs:

CHARMM FF:                                       -0.579 Hartree
DFT-D3:                                                -1.056 Hartree
DFT-D3 with BSSE correction:           -0.968 Hartree

It is quite strange that the value of CHARMM is nearly one half of DFT results.

The binding energy is defined here as: dE = E_bilayer - 2*E_single_layer.

All my input files are attached in the tar file. Please help me to find out where the problem lies in. Thank you!

Best wishes,
Robert

[hut...@chem.uzh.ch]

Hi

- first question: What is the value given in literature?
I'm sure there are others that have calculated this.

- Are your values based on a single geometry (and stacking) or
did you optimize geometry and cell size?

- Your relative cutoff is rather low (25).

Without further information we don't know where to look for
a problem.

regards

Juerg
--------------------------------------------------------------
Juerg Hutter                         Phone : ++41 44 635 4491
Institut für Chemie C                FAX   : ++41 44 635 6838
Universität Zürich                   E-mail: hut...@chem.uzh.ch
Winterthurerstrasse 190
CH-8057 Zürich, Switzerland
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From: Robert Yang
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Date: 06/18/2014 09:24PM
Subject: [CP2K:5421] Binding energy of graphene bilayer: CHARMM vs. DFT-D3

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[attachment "t.tar" removed by Jürg Hutter/at/UZH]

[Robert Yang]

Dear Juerg,

thank you for your response.

On Friday, June 20, 2014 4:47:27 PM UTC+2, jgh wrote:

Hi

- first question: What is the value given in literature?
  I'm sure there are others that have calculated this.

The values given by other DFT calculation are around 40 meV/atom. But it is not clear whether they normalized the total binding energy by the total number of atoms in both layers or in a single layer. If normalized by the atom number in bi-layer, my DFT result is close to reported results. If normalized by atom number in a single layer, my classical result sounds to be correct.
 

- Are your values based on a single geometry (and stacking) or
  did you optimize geometry and cell size?

 

Actually I had done more calculations, for instance the dependence of the binding energy on the vertical distance between the two layers, and the variation of the binding energy with the lateral sliding between layers.
 

- Your relative cutoff is rather low (25).

For such a pure carbon system, would this value of relative cutoff be enough?  I am going to do other simulations with higher cutoff, would 100 be OK?

But I am wondering the cutoff value will give such a big influence, the current DFT result is two times of that of classical FF calculation.

Thanks again for the response!

Best wishes,
Robert

[Leopold Talirz]

Hi Robert,

The values given by other DFT calculation are around 40 meV/atom. But it is not clear whether they normalized the total binding energy by the total number of atoms in both layers or in a single layer. If normalized by the atom number in bi-layer, my DFT result is close to reported results. If normalized by atom number in a single layer, my classical result sounds to be correct.

The binding energy will be given per atom of the single layer. I suggest you have a look to Table 1 of this article in order to compare to the results of your calculations.

 

  http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3818654/ 

Best,

Leo