[Wien] EFG: theory Vs experiment for a case

[Lyudmila Dobysheva]

> From: Ashwani Kumar Friday, December 14, 2018 6:33
> Exper. Calculated : 1.69 x 10^21 V/m2 whereas wien2k calculation: -1.66 x 10^21 V/m2 (crude value still have to do lapw2 -efg) on probe atom. Wien2k calculation shows negative value. is there any significance of the negative sign

> Friday, December 14 2018, 21:40 +04:00 from Stefaan Cottenier <stefaan.cottenier at ugent.be>:
> Most EFG experiments measure the absolute value only, hence you usually do not know the experimental sign.
> The meaning of the sign is the same as for any derivative: a negative Vzz means that d Ez/dz is negative: the value of the z-component of the electric field becomes * smaller *
>  if you go along the positive z-axis.

An addition to what Stefaan said:
According to Laplas equation Vxx+Vyy+Vzz=0, EFG is the biggest among them, and it is chosen as Vzz. So it is not z-axis of global or local coordinate system, it is the biggest among the three values. Vxx is the smallest one. Now imagine that in our system Vxx is almost zero. Then Vzz= - Vyy, they are almost equal. Now, small variations of the system, or of the iterative procedure will give randomly that another axis will be the largest, computer calls this axis z, and you obtain EFG of another sign. 
Look at eta. It is (vyy-vxx)/vzz, and if it is equal to 1 or close, it is this case.

Best regards,
Lyudmila Dobysheva
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