## Negative scattering rates

General discussion around the EPW software

Moderator: hlee

Vahid
Posts: 101
Joined: Fri Apr 08, 2016 11:02 pm
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### Negative scattering rates

Dear EPW users,

When calculating the momentum relaxation time using the following line in the transport.f90 of QE-6.8

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``````vel_factor(ibnd, jbnd) = DDOT(3, vkk(:, ibnd), 1, vkq(:, jbnd), 1) / &
DDOT(3, vkk(:, ibnd), 1, vkk(:, ibnd), 1)           (Eq. 1)
``````
and later

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``````vel_factor(:, :) = one - vel_factor(:, :)    (Eq. 2)
``````
If vkq is larger than vkk, it is possible that Eq. 1 gives a value larger than 1 for certain kpoints.

Eq. 2 then makes the final vel_factor negative which in turn makes the scattering rates for those kpoints negative.

Is there a way these negative rates could be avoided?

One possible solution is to use the following instead of Eq. 1

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``````vel_factor(ibnd,jbnd) = DDOT(3, vkk(:,ibnd), 1,vkq(:,jbnd), 1) / &
(abs(SQRT(DDOT(3, vkk(:,ibnd), 1,vkk(:,ibnd),1)))* &
abs(SQRT(DDOT(3, vkq(:,jbnd), 1,vkq(:,jbnd),1))) )        (Eq. 3)
``````
Eq. 3 will guarantee that vel_factor is never larger than 1.

I would appreciate any input in this matter.

Thank you,
Vahid