Table of Contents

Sensible SCF setups

http://www.cp2k.org

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:

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
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 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!