Atomic Volume

Bader charge density analysis

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Daniel
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Atomic Volume

Post by Daniel »

Dear Sir
I have looked at Atomic volume calculated with Bader Charge analysis and Voronoi analysis using vasp. Atomic volume of individual atoms in water is approximately 100~400 for Oxygen atom in Voronoi but Bader charge analysis volume is about 50 or less than 50 for same oxygen. This is using vasp output. Am I suppose to see that much difference in volume using 2 different partitions? Thank you very much for your input.
graeme
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Re: Atomic Volume

Post by graeme »

For an isolated water molecule, the atomic volumes will have essential no meaning. All of the volume in the calculation will be partitioned to the atoms -- the vast majority of it vacuum.
Daniel
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Joined: Mon Oct 17, 2016 4:03 pm

Re: Atomic Volume

Post by Daniel »

Dear Sir
I got it. Thank you very much. I am interested in the very small change in the atomic charge so I wanted to make sure my understanding is correct. Typically, the change of atomic charge in the order of 0.01. To test my understanding, I have created water molecule in (12A,12A,12A) unit cell in Gaussian. Cube files are using 200pts^3. To compare Voronoi & Bader option, I added total charge since total charge is same for both methods. Interestingly, the output of the screen state 7.99723 for Bader analysis which is exactly same as Voronoi output. However, the sum of the charges in the corresponding ACF.dat is somewhat different. Using ACF.dat, I got 7.075209,0.422591,0.413729 for Bader. For Voronoi, I have 5.3759,1.3029,1.3184 in the output of the screen. Total charge in both case is 7.911529 for Bader and 7.9972 for Voronoi using the same cube file. I assume I should get total charge of 8 or 10 for water. The difference in both partition is about 1 digits after decimal using only 8 million points. This difference seems to much well with the Vaccum charge of 0.0857. That vacuum charge is not partitioned in the Bader analysis but counted in the Voronoi. Am I understanding it correctly? Thank you very much for your comment in advance.
graeme
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Re: Atomic Volume

Post by graeme »

Yes, that's right. For accurate results turn off the vacuum charge option (-vac off). The total charge should then be the same for both methods. But there is no reason to consider the Voronoi partitioning; it is only an option in the code as a sanity check.

Also consider using the '-b weight' for more accurate charges and faster convergence of the Bader charges with respect to the grid density.
Daniel
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Joined: Mon Oct 17, 2016 4:03 pm

Re: Atomic Volume

Post by Daniel »

Dear Sir
Thank you very much. It worked out!!
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