cutoff value optimization question

[Evgeniy Gr]

Hi! I am a beginner in CP2K and I encountered with such problem: when I 
carried out the test CUTOFF value optimization in single point mode the 
total energy has significant changes even at large CUTOFF values (350-500 
Ry). The REL_CUTOFF value is constant (60 Ry). For test example I chose the 
small maleimide molecule. I tried to enlarge the cell volume (ABC = 30, 30, 
30), but for larger cell I obtained the same picture for TOTAL_E changes. 
What am I doing wrong? I will be very grateful for your advices. My 
ultimate goal is to study the mechanisms of organic reactions in a solvent 
medium by DFT molecular dynamics.
CUTOFF (Ry) TOTAL_E
50 -64.842624462853280
150 -66.596463098602229
200 -66.596086697673911
250 -66.593017494126642
300 -66.591148878993607
350 -66.590341443197801
400 -66.590268413708017
450 -66.590267280865888
500 -66.590221265788742



Input File:
&GLOBAL
  PROJECT maleimide-CUT50
  RUN_TYPE ENERGY
  PRINT_LEVEL MEDIUM
&END GLOBAL
&FORCE_EVAL
  METHOD Quickstep
  &SUBSYS
    &KIND C
      ELEMENT   C
      BASIS_SET DZVP-GTH-q4
      POTENTIAL GTH-BP-q4
    &END KIND
    &KIND N
      ELEMENT   N
      BASIS_SET DZVP-GTH-q5
      POTENTIAL GTH-BP-q5
    &END KIND
    &KIND O
      ELEMENT   O
      BASIS_SET DZVP-GTH-q6
      POTENTIAL GTH-BP-q6
    &END KIND
    &KIND H
      ELEMENT   H
      BASIS_SET DZVP-GTH-q1
      POTENTIAL GTH-BP-q1
    &END KIND
    &CELL
      A     15.00000000    0.000000000    0.000000000
      B     0.000000000    15.00000000    0.000000000
      C     0.000000000    0.000000000    15.00000000
    &END CELL
    &COORD
      C   -1.34450  -0.60000   0.00000 
      C   -1.34450   0.75700   0.00000 
      C   -0.02500  -0.91990   0.00000 
      C   -0.02500   1.07680   0.00000 
      N    0.91900   0.07840   0.00000 
      O    0.31260  -2.08220   0.00000 
      O    0.31270   2.23910   0.00000 
      H   -2.23620  -1.24920   0.00000 
      H   -2.23620   1.40620   0.00000 
      H    1.92530   0.07840   0.00000 
    &END COORD
  &END SUBSYS
  &DFT
    BASIS_SET_FILE_NAME  GTH_BASIS_SETS
    POTENTIAL_FILE_NAME  GTH_POTENTIALS
    &QS
      EPS_DEFAULT 1.0E-9
    &END QS
    &MGRID
      NGRIDS 4
      CUTOFF 50
      REL_CUTOFF 60
    &END MGRID
    &XC
      &XC_FUNCTIONAL BP
      &END XC_FUNCTIONAL
    &END XC
    &SCF
      SCF_GUESS ATOMIC
      EPS_SCF 1.0E-7
      MAX_SCF 1
      &DIAGONALIZATION  ON
        ALGORITHM STANDARD
      &END DIAGONALIZATION
      &MIXING  T
        METHOD BROYDEN_MIXING
        ALPHA 0.4
        BETA 0.5
        NBROYDEN 8
      &END MIXING
    &END SCF
  &END DFT
  &PRINT
    &TOTAL_NUMBERS ON
    &END TOTAL_NUMBERS
  &END PRINT
&END FORCE_EVAL


Evgeniy 
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