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Dear Yambo developers,
we are studying the Yambo code. In particular we are interested in how Yambo calculates the oscillators using symmetries.
According to eq. (39) of ref. ( A. Marini et a., yambo: An ab initio tool for excited state calculations, Computer Physics Communications (2009) ) we would like to understand the following code-line (written in the file src/wf_and_fft/scatter_Bamp.F).
As far as we know, g_rot(qs, 1:isc%ngrho) simply applies the simmetry qs to the G vectors, rotating them. We would like to know what fft_g_table (contained in src/wf_and_fft/fft_setup.F) does.
Thank you,
Sincerely
Marco Govoni and Ivan Marri

Dear Marco Govoni and Ivan Marri,

Well I am not really into these technical details (Andrea is but he cannot answer at this moment), but I will try to answer, I hope it helps.

The subroutine calculates
rhotw(G)=<ib ik_is i_sp| e^[I(Rs(G-G0)).r] | ob ok_os o_sp>

to do this, first it multiplies the two wfs in direct space, then fft transforms this product (resulting in rho_tw_rs) and finally (in the line you pointed out) applies the rotation and subtracts G0 using two mappings:
1. Given the indexes of the symmetry operation and a G vector g_rot gives the index corresponding to the rotated Rs G vector.
2. Then, my understanding is that fft_g_table maps the index corresponding to the (rotated) G vector and the index corresponding to the G0 vector into the index corresponding to the G-G0 vector.

Cheers,

m

Let me only add that when performing and FFT from real to reciprocal space, the G components are stored in the array used by the FFT using an order different from the one used in the code. The table
then, gives the index in the FFT ordering of the component relative to the RL vector (G_{iG}-G_{iGo}). In this way the oscillator that is returned by scatter_Bamp is ordered in the correct way.

Dear developers,

we are studying how the oscillators strengths are implemented in the yambo code.
As pointed out in your Com.Phys.Commun. (A2 appendix), the matrix elements are defined as:

rho_{nm} (k,q,G) = Int_dr psi*_{nk} ( r) e^{i (q+G).r } psi_{mk'} ( r)

where k' = k - q - G_0, k' and k are in the 1BZ, q is in 1QBZ. Int_dr denote an integration in the real space over the unit cell. psi are the Bloch wavefunctions:

psi_{nk} ( r) = e^{i k.r} u_{nk} ( r)

so that

rho_{nm} (k,q,G) = Int_dr u*_{nk} ( r) e^{i (G-G_0).r } u_{mk'} ( r)

by applying symmetries on k and k' : k= R k_{IBZ} and k' = R' k'_{IBZ}
the periodic function u transforms as : u_{nk} ( r) = u_{n R k_{IBZ}} ( r) = u_{n k_{IBZ}} ( R^{-1} r)

as a consequence we have

(*) rho_{nm} (k,q,G) = Int_dr u*_{n k_{IBZ} } ( R^{-1} r) e^{i (G-G_0).r } u_{m k'_{IBZ} } ( R'^{-1} r)

----------------------

The procedure is implemented in Scatter_Bamp.F as follows:

Step #1 :
Load the wavefunctions in real space: u_{n k_{IBZ} } ( r) and u_{m k'_{IBZ} } ( r)

Step #2 :
Use symmetries in real space to evaluate the product :
u*_{n k_{IBZ} } ( R^{-1} r) u_{m k'_{IBZ} } ( R'^{-1} r)
where complex conjugate operations can be removed or added depending on the presence of Time-Reversal (litr and lotr variables)

Step #3 :
Perform a Fourier transformation of this product

Step #4 :
Rotate the G-space using symmetry qs and shift the whole rotated space of a G_0 vector.
Comparing this procedure with eq. (*), we don't understand the meaning of rotation qs, which as far as we know is defined as:

q_{QBZ} = R_{qs} q_{QIBZ}

Are we omitting something?
Thank you in advance.

Sincerely,

Marco Govoni and Ivan Marri

marco.govoni wrote: we are studying how the oscillators strengths are implemented in the yambo code.
As pointed out in your Com.Phys.Commun. (A2 appendix), the matrix elements are defined as:

rho_{nm} (k,q,G) = Int_dr psi*_{nk} ( r) e^{i (q+G).r } psi_{mk'} ( r)

In the appendix it is explained how to move the symmetry acting on k on the right, in order to reduce the number of operations to do in the evaluation of the BS kernel.

This procedure, however, is implemented in the routines the call scatter_Bamp.F and not in the routine itself, where it is calculated the most general oscillator of the type

as a consequence we have
(*) rho_{nm} (k,q,G) = Int_dr u*_{n k_{IBZ} } ( R^{-1} r) e^{i (G-G_0).r } u_{m k'_{IBZ} } ( R'^{-1} r)

Comparing this procedure with eq. (*), we don't understand the meaning of rotation qs, which as far as we know is defined as:

q_{QBZ} = R_{qs} q_{QIBZ}

Are we omitting something?

The operation of symmetry acting on G is due to the fact that all summations on G are rotated by the inverse of the symmetry R_{qs}. For example