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I am calculating the second harmonic generation with vasp and have trouble converting the unit.

How to connect the a.u. unit used in VASP to the SI unit?

For example, I noticed in constant.inc, it says:
-------------------------------------------------------------------
! FELECT = (the electronic charge)/(4*pi*the permittivity of free space)
! in atomic units this is just e^2
FELECT = 2*AUTOA*RYTOEV
-------------------------------------------------------------------

Why is this the case?

What if I want to calculate the atomic unit of e^3?

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