What code do you use? If you have a periodic system and use the diagonalization codes then you will already get the spectra momentum resolved.

> On Sep 7, 2014, at 9:35, hamid mosadegh <hamid.mosadegh@gmail.com> wrote:
>
> Dear All
> What is the momentum of , E_1(N_{\uparrow},(N_{\downarrow} ,L), the lowest-excitation energy in the Hubbard model in the S=0 sector.
>
> I means, I want to evaluate the velocity of charge excitation from
>
> E_1(N_{\uparrow},(N_{\downarrow} ,L)- E_0(N_{\uparrow},(N_{\downarrow} ,L)
>
> where E_1(N_{\uparrow},(N_{\downarrow} ,L) is the lowest-excitation energy with
> momentum k=2\pi/L and total spin S=0.
>
> If i run
>
> MODEL="fermion Hubbard"
> CONSERVED_QUANTUMNUMBERS="Nup,Ndown"
> Ndown_total=10;Nup_total=10
> NUMBER_EIGENVALUES=2
>
> what is the momentum of first excitation. is it possible to determine or change it?
>
> thanks
>
>
>
> H. Mosadeq
> Shahr-e-Kord University (SKU),
> Shahr-e-Kord , Iran
>