# Write density on grid of my choice

Sascha Brück bru... at iis.ee.ethz.ch
Tue Apr 15 18:08:44 CEST 2014

Hi,

this would be an idea, however, i just noticed that this is not useful for
me because i do not want to be bound to cp2k writing out data as i process
the density matrix in my own code, so i need the information about how to
evaluate the density from the density matrix. I tried the above formula
with the norms given in the output file but it seemed not to give sensible
results.

I hope I can get some more information about the basis functions and how to
use them to compute the density.

Kind regards
Sascha

Am Dienstag, 15. April 2014 17:12:34 UTC+2 schrieb Matthias Krack:

Hi Sascha,
>
> cp2k calculates the density at the FFT grid points. You may calculate the
> density yourself as you describe, but it is possibly easier to use a fine
> grid by specifying a relatively large cutoff and then to read the cube file
> with an external program which evaluates the density at the points you are
> interested in by (trilinear) interpolation.
>
> Matthias
>
> On Tuesday, April 15, 2014 3:11:44 PM UTC+2, Sascha Brück wrote:
>>
>> Hi,
>>
>> in cp2k, i can write out the density into a cube file, but it will be
>> evaluated at grid points computed by cp2k. If I like to use my own grid
>> with different points where I want to evaluate the density, is there a way
>> to read in that information?
>>
>> If not, it should be easy to evaluate the density by oneself by just
>> summing the density matrix and the basis functions
>>
>> rho(x_grid,y_grid,z_grid)=sum_{i,j} P_{i,j} * phi_i
>> (x_grid-x_atom(i),y_grid-y_atom(i),z_grid-z_atom(i)) * phi_j
>> (x_grid-x_atom(j),y_grid-y_atom(j),z_grid-z_atom(j))
>>
>> where phi_i (x,y,z)=norm_i * pow(x,lx_i) * pow(y,ly_i)* pow(z,lz_i) *
>> exp( -alpha_i*(x^2+y^2+z^2));
>>
>> all coordinates are in bohr radius and alpha_i is from the basis file and
>> the angular momenta are ordered like 1s 1px 1py 1pz 2s 2px 2py 2pz ...
>>
>> Is this formula right? Or do I have to transform to spherical basis
>> functions (even for s and p)? How do i compute the norm?
>>
>> Best regards
>> Sascha
>>
>
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