Content of *.voronoi file

[Hana]

Dear CP2K community,

I have used the voronoi integration feature of CP2K to dump information related to charge centers and dipole moments. In the *.voronoi file I notice that there are "Position", "Charge", "ChargeCenter" and "Dipole" columns. For a specific atom, I want to compare the "Dipole" value with the "Charge" * ("Position"+"ChargeCenter") value. However, they turn out to be quite different. Does anyone have a suggestion on this?

Thanks!

[Fangyong Yan]

Dear Hana,

Dipole moment for  a neutral molecule or molecular system including ionic liquids with the total charge = 0, is invariant from your choice of coordinate systems (you shift your original coordinate center from (0,0,0) to (a,b,x)), the dipole moment does not change.

However, this does not apply to charge species. So if you calculate the dipole moment of an Emim cation, it does not have strict physical meaning because once you change your center to a new center, the dipole moment changes. However, if you put a cation and an anion together, then the dipole moment is meaningful. 

On the other hand, the time derivative of dipole moment, is also invariant to the choice of coordinate system. 

Best regards,

Fangyong

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[Fangyong Yan]

Dear Hana,

I may misunderstand your question, but I think if you use Wannier function, you have the Wannier center for electrons (molecular orbitals); however, if you use voronoi method, you dont have the charge center for electrons, the charge density for voronoi is the summation contribution from all molecular orbitals (the square of molecular orbitals is the probability density for electrons). 

Best regards,

Fangyong

[Fangyong Yan]

So basically if you use electron density, you don’t have center for electrons; if you use wannier center, you have molecular orbital, and you define the center. But in practical, you can say all the centers are indistinguishable because electrons are indistinguishable.

Fangyong 

[Fangyong Yan]

Dear Hana,

For the current, the time derivative of the dipole moment, for charged species, if the charge does not change like classical force field, then the current is invariant to the choice of reference coordinates. 

However, if the charge (electron charge density)  varies, such as in the DFT calculation, then for charged species, the current is no longer invariant to the reference coordinate. 

Best regards,

Fangyong