# Pulay mixing inversion failed. Using only last step.

[serhan]

Hi Nordland,

Could you please briefly explain the difference between SCF current and linear response current?

Serhan

[Nordland]

Well I can give it a try.

Linear Response Current
When you calculate the linear response current, you only perform a self-consistent calculation of the system at zero bias,
and from this calculation you calculate the transmission spectrum T(E). The current is then calculated by performing
an integral between the two chemical levels (adjusted for the voltage) of the electrodes of the transmission and
this you is linear response current.

• Alot faster! (only one SCF calculation )
• Good convergence ( no trouble converging calculations with a bias )
• Limited domain of good accuracy
• Unable to show advanced features of the current curve - like spin-torque in FeMgOFe

SelfConsistent Current.
The effective potential of the twoprobe takes into account that the bias applied has given a voltage ramp in the potential, which
leads to a new hamiltonian, which leads to a new NEGF function, which leads to a new density matrix, which produces a new effective potential.
So this process is continued until there is convergence - just like normal DFT. This selfconsistent process is the essences of ATK, in my eyes.
Once converged, the transmission spectrum is calculated for this state, and the same integral as above is performed and the current is
calculated.

• Very high accuracy
• Results takes into account the perturbation of the system created by the bias.
• Can reveal key-features of the systems
• Calculations time is substantial longer ( many SCF loops)
• Prone to convergence troubles

Which to choose?
It is hard to put a 100% guideline for which to choose, but if I have to an investigation of some materials,
I always first use the selfconsistent approach to see for some relevant sample systems, and compare the IV-curve with
the one obtained from the linear response. If the agreement is acceptable within the bias windows of my study,
then I stick with the linear response current.
However if the system is Magneto-tunnel Junction and I want to model the spin-current through the insulator layers, then I always use the
selfconsistent current.

I hope this answered your question

[serhan]

Thanks Nordland. It was very very helpful