===== Sensible SCF setups =====
http://www.cp2k.org
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===== SCF =====
The Kohn-Sham equations are non-linear. The potential changes as we optimise the Molecular Orbitals.
In this exercise we will explore a few of the options available to get a fast and robust optimisation for most systems.
===== Traditional Diagonalisation (TD) =====
Here is a basic input for a periodic box of water including a SCF section suitable for TD:
&FORCE_EVAL
METHOD QS
&DFT
BASIS_SET_FILE_NAME GTH_BASIS_SETS
BASIS_SET_FILE_NAME BASIS_MOLOPT
POTENTIAL_FILE_NAME POTENTIAL
&MGRID
CUTOFF 300
&END MGRID
&QS
EPS_DEFAULT 1.0E-12
&END QS
&SCF
SCF_GUESS ATOMIC
# SCF_GUESS RESTART
EPS_SCF 1.0E-5
&MIXING
ALPHA 0.4
&END
&END SCF
&XC
&XC_FUNCTIONAL Pade
&END XC_FUNCTIONAL
&END XC
&END DFT
&SUBSYS
&CELL
ABC 9.8528 9.8528 9.8528
&END CELL
# 32 H2O (TIP5P,1bar,300K) a = 9.8528
&COORD
O 2.280398 9.146539 5.088696
O 1.251703 2.406261 7.769908
O 1.596302 6.920128 0.656695
O 2.957518 3.771868 1.877387
O 0.228972 5.884026 6.532308
O 9.023431 6.119654 0.092451
O 7.256289 8.493641 5.772041
O 5.090422 9.467016 0.743177
O 6.330888 7.363471 3.747750
O 7.763819 8.349367 9.279457
O 8.280798 3.837153 5.799282
O 8.878250 2.025797 1.664102
O 9.160372 0.285100 6.871004
O 4.962043 4.134437 0.173376
O 2.802896 8.690383 2.435952
O 9.123223 3.549232 8.876721
O 1.453702 1.402538 2.358278
O 6.536550 1.146790 7.609732
O 2.766709 0.881503 9.544263
O 0.856426 2.075964 5.010625
O 6.386036 1.918950 0.242690
O 2.733023 4.452756 5.850203
O 4.600039 9.254314 6.575944
O 3.665373 6.210561 3.158420
O 3.371648 6.925594 7.476036
O 5.287920 3.270653 6.155080
O 5.225237 6.959594 9.582991
O 0.846293 5.595877 3.820630
O 9.785620 8.164617 3.657879
O 8.509982 4.430362 2.679946
O 1.337625 8.580920 8.272484
O 8.054437 9.221335 1.991376
H 1.762019 9.820429 5.528454
H 3.095987 9.107088 5.588186
H 0.554129 2.982634 8.082024
H 1.771257 2.954779 7.182181
H 2.112148 6.126321 0.798136
H 1.776389 7.463264 1.424030
H 3.754249 3.824017 1.349436
H 3.010580 4.524142 2.466878
H 0.939475 5.243834 6.571945
H 0.515723 6.520548 5.877445
H 9.852960 6.490366 0.393593
H 8.556008 6.860063 -0.294256
H 7.886607 7.941321 6.234506
H 7.793855 9.141028 5.315813
H 4.467366 9.971162 0.219851
H 5.758685 10.102795 0.998994
H 6.652693 7.917443 3.036562
H 6.711966 7.743594 4.539279
H 7.751955 8.745180 10.150905
H 7.829208 9.092212 8.679343
H 8.312540 3.218330 6.528858
H 8.508855 4.680699 6.189990
H 9.742249 1.704975 1.922581
H 8.799060 2.876412 2.095861
H 9.505360 1.161677 6.701213
H 9.920117 -0.219794 7.161006
H 4.749903 4.186003 -0.758595
H 5.248010 5.018415 0.403676
H 3.576065 9.078451 2.026264
H 2.720238 9.146974 3.273164
H 9.085561 4.493058 9.031660
H 9.215391 3.166305 9.749133
H 1.999705 2.060411 1.927796
H 1.824184 0.564565 2.081195
H 7.430334 0.849764 7.438978
H 6.576029 1.537017 8.482885
H 2.415851 1.576460 8.987338
H 2.276957 0.099537 9.289499
H 1.160987 1.818023 4.140602
H 0.350256 2.874437 4.860741
H 5.768804 2.638450 0.375264
H 7.221823 2.257514 0.563730
H 3.260797 5.243390 5.962382
H 3.347848 3.732214 5.988196
H 5.328688 9.073059 5.982269
H 5.007063 9.672150 7.334875
H 4.566850 6.413356 3.408312
H 3.273115 7.061666 2.963521
H 3.878372 7.435003 6.843607
H 3.884673 6.966316 8.283117
H 5.918240 3.116802 5.451335
H 5.355924 2.495093 6.711958
H 5.071858 7.687254 10.185667
H 6.106394 7.112302 9.241707
H 1.637363 5.184910 4.169264
H 0.427645 4.908936 3.301903
H 9.971698 7.227076 3.709104
H 10.647901 8.579244 3.629806
H 8.046808 5.126383 2.213838
H 7.995317 4.290074 3.474723
H 1.872601 7.864672 7.930401
H 0.837635 8.186808 8.987268
H 8.314696 10.115534 2.212519
H 8.687134 8.667252 2.448452
&END COORD
&KIND H
BASIS_SET DZVP-GTH
POTENTIAL GTH-PADE-q1
&END KIND
&KIND O
BASIS_SET DZVP-GTH
POTENTIAL GTH-PADE-q6
&END KIND
&END SUBSYS
&END FORCE_EVAL
&GLOBAL
PROJECT H2O-32
RUN_TYPE MD
PRINT_LEVEL MEDIUM
&END GLOBAL
**TASK**
This uses a rather small BASIS_SET DVZP-GTH which is much too small for production runs.
Repeat the calculation using
BASIS_SET TZV2P-GTH
BASIS_SET DZVP-MOLOPT-GTH
BASIS_SET TZV2P-MOLOPT-GTH
You should change the basis set for each atomic type (kind) in each case.
===== Orbital Transformation (OT) =====
We can see the effect of changing to the OT method by simply changing the SCF section.
&SCF
SCF_GUESS ATOMIC
EPS_SCF 1.0E-06
MAX_SCF 20
&OT ON
MINIMIZER DIIS
PRECONDITIONER FULL_ALL
ENERGY_GAP 0.001
&END OT
&OUTER_SCF
MAX_SCF 2
&END OUTER_SCF
&END SCF
**TASK**
See how OT compares with TD for the different basis sets you ran previously.
=====Other parameters=====
There are other factors that influence the effectiveness of the setups.
**TASK**
Exchange correlation functional.
We were using LDA (the particular parameterization is called PADE in CP2K). Try changing the functional to `PBE` (in the `XC` section) and see how this changes convergence.
Also change the pseudopotential you are using in the KIND sections so it matches the functional.
**TASK**
System size affects the efficiency too. You can find larger water boxes in the ''${main directory of my cp2k installation}/tests/QS/benchmarks'' directory, or {{https://sourceforge.net/p/cp2k/code/HEAD/tree/trunk/cp2k/tests/QS/benchmark/|online}}.
Change the CELL parameters and the coordinates of the atoms and see how the methods scale.
You could also explore how the methods scale with number of processors used.
**TASK**
And of course, probably the most important 'parameter' is the system itself. Water is quite well behaved - largely meaning it is closed shell and has a large band gap. Try similar tests on a system you are interested in!