[CP2K-user] [CP2K:15386] Unreasonable CELL_OPT result obtained for TiO2

[Krack Matthias (PSI)]

Hi

I guess you have been using DFT-PBE as level of theory for fcc Cu. In that case, a deviation of only 0.03 A for the relaxed lattice constant compared to experiment seems to me acceptable. DFT-GGA is known to give often too large lattice constants.

M.

Von: cp... at googlegroups.com <cp... at googlegroups.com> Im Auftrag von Hasan Al-Mahayni
Gesendet: Sonntag, 16. Mai 2021 18:54
An: cp... at googlegroups.com
Betreff: Re: [CP2K:15386] Unreasonable CELL_OPT result obtained for TiO2

Hi,

I’m gonna jump in if you don’t mind. I am trying to do cell optimization of  fcc copper. I used k points and got 3.65A  which is different from the 3.62 that was expected. I was wondering for your latter option (using a large simulation cell) , would Run CELL_OPT or GEO_OPT?

On Sun, May 16, 2021 at 8:46 PM Krack Matthias (PSI) <matthi... at psi.ch<mailto:matthi... at psi.ch>> wrote:
Hi Yike

Your simulation cell is small and you use no k point sampling. Either you use k point sampling or you enlarge your simulation cell to at least a cell edge length of 12 A in each direction. I suggest the latter option for CP2K. Furthermore, you could reduce EPS_DEFAULT to 1.0E-12 and increase REL_CUTOFF to 60.

HTH

Matthias

Von: cp... at googlegroups.com<mailto:cp... at googlegroups.com> <cp... at googlegroups.com<mailto:cp... at googlegroups.com>> Im Auftrag von Yike Huang
Gesendet: Sonntag, 16. Mai 2021 18:23
An: cp2k <cp... at googlegroups.com<mailto:cp... at googlegroups.com>>
Betreff: [CP2K:15385] Unreasonable CELL_OPT result obtained for TiO2

Hello, cp2k developers and users,

I try to optimize primitive cell of rutile TiO2 by running CELL_OPT calculation at PBE+U(presently set to 0 eV)/DZVP level, but I find cell parameters deviate from experimental values quite quickly during calculation.
I have tried changing STRESS_TENSOR to both NUMERICAL and ANALYTICAL but it does not help, I also think it may be not the reason why I get this kind of strange result.
I attach my input script and CIF file below, are there any parameters in input script I forget?
In addition, anatase TiO2 CELL_OPT task also has such problem, but I remember that primitive cell of Co3O4 can be successfully optimized.

Thanks in advance.

Yike Huang
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