[CP2K-user] [CP2K:20444] Why does the energy of the lowest band not decrease monotonously when increasing size of basis sets?

[Yike Huang]

Dear all,

I am working on testing excitation of very simple molecules such as Ag 
dimer, and examining both occupied and unoccupied states calculated. 
However, I find when I change the basis from SVP to TZVPP, the energy of 
the lowest band increases a littble bit, then decreases if I furtherly 
change to QZVPP basis (all basis are from BASIS_MOLOPT_UZH collection, I 
set CUTOFF to 600 Ry, REL_CUTOFF 30 Ry):

basis     SVP       TZVPP     QZVPP
e(1)    -96.3005 -95.3219 -96.1675

energies are in eV.

To my knowledge if the completeness of basis set increases, the energy 
should be lower.
I also convert the pseudopotential to UPF file (by CP2K ATOM module) and 
perform pw calculation with Quantum ESPRESSO, I set the kinetic energy 
cutoff to 300 Ry but still find the energy is higher than any of what I 
obtained from CP2K. But in principle pw is a very huge basis set, so I get 
confused again...

basis      PW
e(1)    -94.8071

Very best wishes,
Yike

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