I try to compute the unit cell energy of the $\ce{Ti2CO2}$ MXene with NWchem.
Computing the 1x1 supercell with 4x4x1 K-points grid I got -1875 Ha.
Computing the 2x2 supercell with 2x2x1 K-points grid I got -7502 Ha, that means -1875.5 Ha per unit cell.
I understand, that for useful results convergence should be reached with K-points and the basis set. But here I just want to understand the source of the 0.5 Ha discrepancy. Previously I thought, that NxM supercell is equivalent to NxM K-points grid. Apparently it is not true. What is the source of such a significant difference?
Here are the NWchem inputs:
1x1 supercell with 4x4x1 K-points
title "nwtest"
permanent_dir /home/pvst/prg/regr/nwtest
scratch_dir /home/pvst/prg/regr/nwtest
start nwtest
geometry units angstrom nocenter noautosym noautoz
system crystal units angstrom
lattice_vectors
3.0136139399999999e+00 0.0000000000000000e+00 0.0000000000000000e+00
-1.5068069700000000e+00 2.6098661399999998e+00 0.0000000000000000e+00
0.0000000000000000e+00 0.0000000000000000e+00 3.0000000000000000e+01
end
Ti 3.4287223190009325e-01 6.5239607890387818e-01 2.1337608300333333e-01
Ti 6.7638044023217136e-01 3.1914803561534388e-01 1.2846060203666668e-01
C 9.5644354566451414e-03 9.8572931602538050e-01 1.7248868730000000e-01
O 6.7625413136791823e-01 3.1911195292184602e-01 2.4912464496666664e-01
O 3.4304300093474527e-01 6.5247955598213170e-01 9.7693002513333335e-02
end
basis noprint
* library 3-21g
end
dft
maxiter 2000
xc revpbe
mult 1
end
nwpw
monkhorst-pack 4 4 1
mult 1
end
task dft gradient
2x2 supercell with 2x2x1 K-points
title "nwtest"
permanent_dir /home/pvst/prg/regr/nwtest
scratch_dir /home/pvst/prg/regr/nwtest
start nwtest
geometry units angstrom nocenter noautosym noautoz
system crystal units angstrom
lattice_vectors
6.0272278799999999e+00 0.0000000000000000e+00 0.0000000000000000e+00
-3.0136139399999999e+00 5.2197322799999997e+00 0.0000000000000000e+00
0.0000000000000000e+00 0.0000000000000000e+00 3.0000000000000000e+01
end
Ti 1.7143611595004662e-01 3.2619803945193909e-01 2.1337608300333333e-01
Ti 3.3819022011608568e-01 1.5957401780767194e-01 1.2846060203666668e-01
C 4.7822177283225707e-03 9.9286465801269030e-01 1.7248868730000000e-01
O 3.3812706568395912e-01 1.5955597646092301e-01 2.4912464496666664e-01
O 1.7152150046737263e-01 3.2623977799106585e-01 9.7693002513333335e-02
Ti 1.7143611595004657e-01 8.2619803945193904e-01 2.1337608300333333e-01
Ti 3.3819022011608568e-01 6.5957401780767189e-01 1.2846060203666668e-01
C 4.7822177283225655e-03 4.9286465801269025e-01 1.7248868730000000e-01
O 3.3812706568395912e-01 6.5955597646092301e-01 2.4912464496666664e-01
O 1.7152150046737266e-01 8.2623977799106596e-01 9.7693002513333335e-02
Ti 6.7143611595004660e-01 3.2619803945193909e-01 2.1337608300333333e-01
Ti 8.3819022011608557e-01 1.5957401780767194e-01 1.2846060203666668e-01
C 5.0478221772832266e-01 9.9286465801269030e-01 1.7248868730000000e-01
O 8.3812706568395923e-01 1.5955597646092301e-01 2.4912464496666664e-01
O 6.7152150046737269e-01 3.2623977799106585e-01 9.7693002513333335e-02
Ti 6.7143611595004660e-01 8.2619803945193904e-01 2.1337608300333333e-01
Ti 8.3819022011608579e-01 6.5957401780767189e-01 1.2846060203666668e-01
C 5.0478221772832255e-01 4.9286465801269025e-01 1.7248868730000000e-01
O 8.3812706568395923e-01 6.5955597646092301e-01 2.4912464496666664e-01
O 6.7152150046737269e-01 8.2623977799106596e-01 9.7693002513333335e-02
end
basis noprint
* library 3-21g
end
dft
maxiter 2000
xc revpbe
mult 1
end
nwpw
monkhorst-pack 2 2 1
mult 1
end
task dft gradient