Messages

1

General Questions and Answers / Re: Voltage symmetry

« on: July 16, 2009, 18:07 »

Thanks for the explanation. Perhaps you can clarify for me how the voltages are referenced. If only the potential difference matters, one could specify for instance, electrode_voltages = (11.0,10.0)*Volt and this would be equivalent to electrode_voltages = (1.0,0.0)*Volt. Is this the case? If so, why specify electrode voltages at all, and not just the potential difference?

For my purposes, it would be useful to have the voltages each referenced individually to something in particular. I had thought this reference would be the Fermi level of the unbiased electrodes. If this were the case, electrode_voltages = (1.0,1.0)*Volt would have a physical meaning (that is, the potential at both electrodes would be shifted compared to the original Fermi level), and would not yield the same result as electrode_voltages = (0.0,0.0)*Volt. Is fixing the reference possible?

2

General Questions and Answers / Voltage symmetry

« on: July 15, 2009, 22:01 »

Hi all,

I have a question regarding the meaning of setting the voltages in ATK. It seems from Phys Rev B 63, 245407(2001) that each electrode is set to have a potential that is bulk + V, with 2 different V's, one for each electrode. So I would guess that

electrode_voltages = (-0.1,0.0)*Volt and electrode_voltages = (0.0,0.1)*Volt

should have the same potential drop from left to right, but the results should not be equivalent for an asymmetric system. However, upon trying this, I found that the charges on the atoms converge to the same state. My system is simply two identical metal electrodes with a molecule absorbed on one of the two electrodes.

With electrode_voltages = (-0.1,0.0)*Volt:
# sc 17 : q =  195.88425 e  Etot = -4000.99030 Ry  dRho =  4.2499E-04  dEtot =  1.6844E-06 Ry

and with electrode_voltages = (0.0,-0.1)*Volt:
# sc 18 : q =  195.88463 e  Etot = -4000.98381 Ry  dRho =  1.9519E-04  dEtot = -3.8440E-06 Ry

Although the system is assymetric, both calculations converge to the same state -- as if only the relative voltage drop mattered. Please help explain this phenomenon. Is this the expected behavior? If so, how would one fix an electrode potential to capture the asymmetry?

Thanks!

3

General Questions and Answers / total charge question

« on: July 6, 2009, 23:14 »

Hello,

I am wondering about the quantity "Charge" in the properties output. I ran a twoprobe calculation, which converged to:

# sc 26 : q =  193.26029 e  Etot = -3687.65468 Ry  dRho =  2.6318E-04  dEtot =  7.0024E-06 Ry

and then the properties section says:

# ----------------------------------------------------------------
# Total Energy           = -26398.6064 eV             
# Potential Energy       = -26398.6064 eV             
# Ion Kinetic Energy     = 0.0000 eV
# ----------------------------------------------------------------             
# NumOrb: Energy of system
# ----------------------------------------------------------------             
# Charge                  = 195.1892
# Band Structure Energy   = -726.1524 eV               
# ----------------------------------------------------------------

The final iteration giving charge is not equal to the charge given in the properties output. Any explanation would be helpful.

Thanks!
--Paul