[CP2K-user] [CP2K:16851] What are the default units of hartree potential and electric field in cp2k?

[gary Washington]

I find that we can test for the units of the Hartree potential by using a 
test molecule. I selected Li2 and applied an external electric field 
(Volts/Angstroms). First obtain the Hartree Potential cube file for a zero 
external potential

# electric field along the z direction at efieldmag eV/Angstrom or 
efieldmag V/Angstrom
    &EXTERNAL_POTENTIAL
      FUNCTION (A/B)*Z
      VALUES [eV] ${efieldmag} [angstrom] 1.0
      PARAMETERS A B
    &END EXTERNAL_POTENTIAL


Cutoff = 400, RelCutoff=80
so we set efieldmag = 0.0

Now using a box/cell **without periodic boundary conditions** we can 
optimize the geometry ( an energy calculation should also work ) now we use 
cubecruncher to obtain the potential along the z axis. We expect the 
initial slope, which is equal to the initial electric field magnitude in 
the z-direction. Now molecules will have an electric field around them due 
to their charge distribution of nuclei and electrons so even when 
efieldmag=0.0 the electric field around the molecule will be nonzero and 
will diminish as you move away from the molecule. If the box is large 
enough the influence of the molecule on the initial electric field will 
nearly vanish far from the molecule. I used {Lx, Ly, Lz} of {12,12,15} , 
{12,12,20}, {12,12,25} and the initial slopes are -0.019047, -0.00027563, 
-0.0000032. The initial slope is obtained on the left in the limit as z --> 
0. We see the influence of the molecule can be greatly reduced in a 
sufficiently large box and this is expected to be true for any molecule in 
a sufficiently large box/cell. 

Now we set efieldmag=0.005 Volts/Angstrom, using a box with dimensions of 
{12,12,15}, {12,12,20}, {12,12, 25} and the initial slopes are -0.01932522, 
-0.00008413067, -0.0001806136
Next we set Cutoff=600, relCutoff=80 and box size {12,12, 25} and the 
initial slope is -0.0001811421
Next we set Cutoff=800, relCutoff=80  and box size {12,12, 25} and the 
initial slope is -0.0001815272
Next we set Cutoff=800, relCutoff=80  and box size {12,12, 35} and the 
initial slope is -0.0001837458

* assumption 1:                                   Hartree potential has the 
units of hartree/e = 27.211386 Volts / a.u.
*assumption 2:                                    Electric Field Magnitude 
(Volts/Angstroms) = 27.211386 * (Hartree Potential at a point)

Check: Given an electric field magnitude of 0.005 V/Ang we expect that the 
observed Hartree potential sufficiently far from the molecule will yield a 
slope along the z axis, Ez (electric field in the z direction) of the same 
magnitude.                We expect that the result should approach (0.005 
V/Ang)/27.211386 = 0.000183747 a.u. ( the expected slope of the Hartree 
potential due only to the external potential)

We see that the initial slope is equal to efieldmag/27.211386 (Volts/a.u.) 
which shows that assumption 1 holds. 





On Monday, March 22, 2021 at 2:25:52 PM UTC-4 fyya... at gmail.com wrote:

> Hi, Kai,
>
> 50000 MV/cm = 50 V / (10E8) Angstrom = 5E-7 V/Angstrom, which is a very 
> very small electric field. 
>
> Best,
>
> Fangyong
>
> On Mon, Mar 22, 2021 at 1:07 PM Kaixuan Chen <kaixu... at gmail.com> wrote:
>
>>
>> Dear all,
>> I have generated the cube files of hartree potential (CP2K_INPUT 
>> <https://manual.cp2k.org/cp2k-8_1-branch/CP2K_INPUT.html> / FORCE_EVAL 
>> <https://manual.cp2k.org/cp2k-8_1-branch/CP2K_INPUT/FORCE_EVAL.html> / 
>> DFT 
>> <https://manual.cp2k.org/cp2k-8_1-branch/CP2K_INPUT/FORCE_EVAL/DFT.html> 
>> / PRINT 
>> <https://manual.cp2k.org/cp2k-8_1-branch/CP2K_INPUT/FORCE_EVAL/DFT/PRINT.html> 
>> / V_HARTREE_CUBE 
>> <https://manual.cp2k.org/cp2k-8_1-branch/CP2K_INPUT/FORCE_EVAL/DFT/PRINT/V_HARTREE_CUBE.html>) 
>> and electric field (CP2K_INPUT 
>> <https://manual.cp2k.org/cp2k-8_1-branch/CP2K_INPUT.html> / FORCE_EVAL 
>> <https://manual.cp2k.org/cp2k-8_1-branch/CP2K_INPUT/FORCE_EVAL.html> / 
>> DFT 
>> <https://manual.cp2k.org/cp2k-8_1-branch/CP2K_INPUT/FORCE_EVAL/DFT.html> 
>> / PRINT 
>> <https://manual.cp2k.org/cp2k-8_1-branch/CP2K_INPUT/FORCE_EVAL/DFT/PRINT.html> 
>> / EFIELD_CUBE 
>> <https://manual.cp2k.org/cp2k-8_1-branch/CP2K_INPUT/FORCE_EVAL/DFT/PRINT/EFIELD_CUBE.html>) 
>> from cp2k. I don't see an explicit description on the units that are used 
>> in these cube files. If I take the a.u. as the default unit (hartree/e for 
>> potential, and hartree/e/bohr for electric field), the value seems pretty 
>> large. For example, I study the single water molecule system. The largest 
>> electric field at some density grid is 10~15 hartree/e/bohr, that is, 
>> ~50000 MV/cm. Please correct me if I am wrong, but the value seems 
>> unreasonable to me.
>> Any suggestion will be welcome, thanks in advance.
>> Best,
>> Kai
>>
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>> .
>>
>

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