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

[Jürg Hutter]

Hi
on what grounds you expect the orbital energies to be variational?
Only the total KS energy will have this property.
regards
JH

________________________________________
From: cp...@googlegroups.com <cp...@googlegroups.com> on behalf of Yike Huang <ykhua...@gmail.com>
Sent: Thursday, July 11, 2024 12:34 PM
To: cp2k
Subject: [CP2K:20444] Why does the energy of the lowest band not decrease monotonously when increasing size of basis sets?

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[Yike Huang]

Thanks for reminding me! Yes I only remember the total energy is variational and for orbital energy I am not quite sure. 

Yike