Convert physical quantity defined on the real fine grid to the reciprocal grid (GRIDC%RC)

[admin]

Hello,

My question is related to converting a quantity from the real to the reciprocal fine grid. I have a physical quantity that is a function of the charge density defined on the real fine grid (GRIDC%RL). That is, I have the charge density on the tridimensional real fine grid (X,Y,Z) and I have calculated another quantity namely P(X,Y,Z) from it. Now, I would like to convert this quantity from this real fine grid (which has GRIDC%NGX elements along the x direction, GRIDC%NGY elements along the y direction, and GRIDC%NGZ elements along the z direction) to the reciprocal fine grid. The main thing is that, in order to deal with FFTs, VASP orders the grid in columns that one can identify with their x-y index in real space and with the y-z index in the reciprocal grid (GRIDC%RC%I2 and GRIDC%RC%I3).
I know, for instance how to convert a charge density from the reciprocal space to the real space (I have done it like in the fileio.F subroutine, that is using the 'RC_ADD', FFT3D' and the 'MRG_GRID_RL_PLANE' subroutines), but I do not know how to convert the same charge density in the real fine tridimensional grid backwards to the reciprocal grid.

Someone could help me in this operation?. I think I would be able to do it, but in order not to forget some intermediate calculation I preferred to use this forum.

Many thanks in advance
<span class='smallblacktext'>[ Edited ]</span>