[CP2K-user] [CP2K:20447] Why does the energy of the lowest band not decrease monotonously when increasing size of basis sets?

[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
在2024年7月12日星期五 UTC+8 15:50:25<Jürg Hutter> 写道：

> 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... at googlegroups.com <cp... at googlegroups.com> on behalf of Yike 
> Huang <ykhua... at 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?
>
> 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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