# 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.

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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