[CP2K-user] OT/Supercell vs. Kpoints: Abnormal pressure difference

[Nicholas Winner]

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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