[CP2K-user] OT/Supercell vs. Kpoints: Abnormal pressure difference
Nicholas Winner
nwi... at berkeley.edu
Fri Apr 2 18:14:00 UTC 2021
Hello all,
I performed two cell optimizations on bulk Si, the first with a primitive
cell and kpoints and the second using OT and a 250 atom supercell. Both
structures started from the Materials Project (mp-149) lowest energy Fm-3m
polymorph.
I found that while OT only took 3 cell opt steps, the kpoint calculation
took 10 steps, which I found a little strange.
Upon investigation I found two things:
(a) the kpoint calculation, had a large initial pressure deviation of
-92570 bar, compared to OT, which had an initial deviation of -708 bar at
DZVP level and -2934 at TZV2PX level. Again these are the same structure,
just kpoints vs supercell. It is a little odd to me that DZVP is closer to
converged pressure, but I don't think its too strange.
(b) the kpoint calculation relaxed from Fm-3m to Im-3m space group, which
is not the ground state polymorph.
My question is whether or not this issue is due to CP2K's kpoint
implementation, or if this is something more fundamental, and how (if at
all) it can be resolved. Below is the abridged DFT section for the kpoint
calculation, which I assume is where the problem lies, but I can provide
more info if needed.
Best,
Nick
&DFT
BASIS_SET_FILE_NAME BASIS_MOLOPT
BASIS_SET_FILE_NAME BASIS_MOLOPT_UCL
POTENTIAL_FILE_NAME GTH_POTENTIALS
UKS True
MULTIPLICITY 0
CHARGE 0
&QS
METHOD GPW
EPS_DEFAULT 1e-12
EXTRAPOLATION PS
&END QS
&SCF
MAX_SCF 100
EPS_SCF 1e-07
SCF_GUESS RESTART
MAX_ITER_LUMO 400
ADDED_MOS 100
&DIAGONALIZATION
&DAVIDSON
PRECONDITIONER FULL_ALL
&END DAVIDSON
&END DIAGONALIZATION
&MIXING
METHOD BROYDEN_MIXING
ALPHA 0.2
NBUFFER 5
&END MIXING
&SMEAR
ELEC_TEMP 300
METHOD FERMI_DIRAC
FIXED_MAGNETIC_MOMENT -100.0
&END SMEAR
&END SCF
&KPOINTS
SCHEME MONKHORST-PACK 13 13 13
EPS_GEO 1e-06
FULL_GRID False
PARALLEL_GROUP_SIZE -1
SYMMETRY False
UNITS B_VECTOR
VERBOSE True
WAVEFUNCTIONS COMPLEX
&END KPOINTS
&END DFT
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