Meaning of total ground state energy in periodic DFT calculations

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In periodic DFT calculations, such as those performed with VASP, the total ground state energy is calculated per unit cell due to the use of periodic boundary conditions that simulate an infinite crystal. The energy value is finite because it represents the energy associated with a single repeating unit of the crystal structure, not the entire infinite lattice. This approach allows for practical calculations while maintaining the properties of the system. Understanding this concept is crucial for interpreting results in the context of solid-state physics. The finite energy value reflects the inherent characteristics of the unit cell rather than suggesting an infinite energy for the entire crystal.
Derivator
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Dear all,

periodic DFT codes (e.g. VASP) effectively simulate an infinite crystal due to the periodic boundary conditions. However, the energy value that one obtaines at the end of a simulation if finite. Frankly, I'm quite confused right now.

Is the energy to be understood 'per unit cell'? Why is the energy value not infinite?

Cheers,
derivator
 
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Yes, it is per unit cell.
 
Thread 'Unexpected irregular reflection signal from a high-finesse cavity'

Thread 'Unexpected irregular reflection signal from a high-finesse cavity'

I am observing an irregular, aperiodic noise pattern in the reflection signal of a high-finesse optical cavity (finesse ≈ 20,000). The cavity is normally operated using a standard Pound–Drever–Hall (PDH) locking configuration, where an EOM provides phase modulation. The signals shown in the attached figures were recorded with the modulation turned off. Under these conditions, when scanning the laser frequency across a cavity resonance, I expected to observe a simple reflection dip. Instead...
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