6
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I was performing DFTB3 calculation with DFTB+ on water monomer. This is my input.

Geometry = xyzformat{
<<< "water.xyz"
}

Driver{}

Hamiltonian = DFTB {
 SCC = Yes
 SCCTolerance = 1.0e-08
 SpinPolarisation = {}
 SlaterKosterFiles {
  O-O = O-O.skf
  H-H = H-H.skf
  H-O = H-O.skf
  O-H = O-H.skf
  }
 MaxAngularMomentum{
  O = p
  H = s
 }
 Filling = Fermi {
  Temperature [Kelvin] = 0.0
 }
 ThirdOrderFull = Yes
 HubbardDerivs {
  O = -0.1575
  H = -0.1857
 }
}
Options={}
ParserOptions {
 ParserVersion = 5
}

The output it shows (I am attaching full output.)

|===============================================================================
|
|  DFTB+ release 21.1
|
|  Copyright (C) 2006 - 2021  DFTB+ developers group
|
|===============================================================================
|
|  When publishing results obtained with DFTB+, please cite the following
|  reference:
|
|  * DFTB+, a software package for efficient approximate density functional
|    theory based atomistic simulations, J. Chem. Phys. 152, 124101 (2020).
|    [doi: 10.1063/1.5143190]
|
|  You should also cite additional publications crediting the parametrization
|  data you use. Please consult the documentation of the SK-files for the
|  references.
|
|===============================================================================

Reading input file 'dftb_in.hsd'
Parser version: 9

--------------------------------------------------------------------------------
***  Converting input from parser version  5 to parser version  9 ...
***  Done.

Reading SK-files:
Done.


Processed input in HSD format written to 'dftb_pin.hsd'

Starting initialization...
--------------------------------------------------------------------------------
OpenMP threads:              8
Chosen random seed:          44487222
Mode:                        Static calculation
Self consistent charges:     Yes
SCC-tolerance:                 0.100000E-07
Max. scc iterations:                    100
Shell resolved Hubbard:      No
Spin polarisation:           No
Nr. of up electrons:             4.000000
Nr. of down electrons:           4.000000
Periodic boundaries:         No
Electronic solver:           Relatively robust
Mixer:                       Broyden mixer
Mixing parameter:                  0.200000
Maximal SCC-cycles:                     100
Nr. of chrg. vec. in memory:              0
Electronic temperature:        0.100000E-07 H      0.272114E-06 eV
Initial charges:             Set automatically (system chrg:   0.000E+00)
Included shells:             O:  s, p
                             H:  s
Full 3rd order correction    Yes
Extra options:
                             Mulliken analysis

--------------------------------------------------------------------------------

***  Geometry step: 0

 iSCC Total electronic   Diff electronic      SCC error    
    1    0.00000000E+00    0.00000000E+00    0.78671399E+00
    2   -0.41164337E+01   -0.41164337E+01    0.58636504E+00
    3   -0.41186098E+01   -0.21761050E-02    0.27136865E-01
    4   -0.41207453E+01   -0.21354711E-02    0.16271916E-02
    5   -0.41207483E+01   -0.29932945E-05    0.22850510E-02
    6   -0.41207483E+01    0.57787499E-08    0.43132836E-04
    7   -0.41207483E+01   -0.21815850E-07    0.90476243E-07
    8   -0.41207483E+01   -0.74171780E-11    0.13073986E-11
>> Charges saved for restart in charges.bin

Total Energy:                       -4.0618820527 H         -110.5294 eV
Extrapolated to 0K:                 -4.0618820527 H         -110.5294 eV
Total Mermin free energy:           -4.0618820527 H         -110.5294 eV
Force related energy:               -4.0618820527 H         -110.5294 eV


--------------------------------------------------------------------------------
DFTB+ running times                          cpu [s]             wall clock [s]
--------------------------------------------------------------------------------
SCC                                    +     0.07  ( 98.6%)      0.01  (100.0%)
--------------------------------------------------------------------------------
Missing                                +     0.00  (  1.4%)      0.00  (  0.0%)
Total                                  =     0.07  (100.0%)      0.01  (100.0%)
--------------------------------------------------------------------------------

Previously I have calculated the same using HF/def2-QZVPPD having energy of ~-76 Hartree. My question is that, How can it be so different from -76 (HF) to -4 (DFTB3)? Am I missing something?

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$\endgroup$
13
  • 1
    $\begingroup$ I am not familiar with this program/method in detail but some parametrized methods leave out the core electrons and only treat the valence electrons and print only the energy of the treated valence system. This can explain a large energy difference like that. Do you have output that lists the energy in more detail ? Where you can see the number of included orbitals and their energies or something similar ? $\endgroup$
    – Hans Wurst
    Commented Sep 13, 2021 at 15:09
  • 1
    $\begingroup$ @Tyberius I have added the full input and output. $\endgroup$
    – Pro
    Commented Sep 13, 2021 at 17:41
  • 2
    $\begingroup$ The output mentions only 8 electrons, (4 up and 4 down), while water should have 10 electrons. It seems that the two oxygen 1s core electrons are not explicitly included. But I am afraid that the output is not particularly informative, so I'm only guessing. $\endgroup$
    – Hans Wurst
    Commented Sep 13, 2021 at 18:01
  • 1
    $\begingroup$ Could the input geometry be in the wrong units? Like Bohr instead of angstroms? Sorry. No insult intended. It's just a very easy mistake to make :) $\endgroup$
    – jheindel
    Commented Sep 21, 2021 at 22:35
  • 1
    $\begingroup$ @jheindel, that thought occurred to me too. I then cross-checked with gmolden. I checked that it was in angstroms. $\endgroup$
    – Pro
    Commented Sep 22, 2021 at 6:10

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