CP2K Open Source Molecular Dynamics

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===== Sensible SCF setups =====

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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:

<code - scf_basic.inp>
&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       
</code>

<note>**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.
</note>
===== Orbital Transformation (OT) =====

We can see the effect of changing to the OT method by simply changing the SCF section.

<code>
&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
</code>

<note>**TASK**
See how OT compares with TD for the different basis sets you ran previously.
</note>

=====Other parameters=====

There are other factors that influence the effectiveness of the setups.

<note>**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.
</note>

<note tip>
Also change the pseudopotential you are using in the KIND sections so it matches the functional.
</note>

<note>**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.
</note>

<note>**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!</note>