Mismatch between CP2K computed Force constant (a.u)and Gaussian computed force constant(mDyne/angstrom)

[Naina Sethi]

Dear all,
I am working on calculating the vibrational frequencies and force constants of a water molecule. In CP2K, the force constants are reported in atomic units (Hartree/Bohr²), while Gaussian lists them in mDyne/Å. The conversion factor between Hartree/Bohr² and mDyne/Å is about 15.57. However, even after applying this factor, the results from CP2K do not match the force constant values reported by Gaussian. Could anyone clarify why this discrepancy occurs?  

       Frc consts --           1.7016                   8.9360               9.7649        (gaussian)

VIB|Frc consts (a.u.)    0.017668               0.567398         0.663392     (CP2K)

Thanks and Regards

Naina

[Frederick Stein]

Dear Naina,

Are you sure that all relevant parameters (especially CUTOFF, EPS_DEFAULT, EPS_SCF) are well converged? Beware that a vibrational analysis requires much tighter thresholds than force calculations. Can you provide your input file? How do the frequencies compare between both codes?

Best,

Frederick

[Naina Sethi]

The frequencies and the reduced masses are very similar in both Gaussian and CP2K.(probably because both of them report in the same units).

I've included for you the CP2k input for frequency cal.

&GLOBAL
  PROJECT H2O
  RUN_TYPE VIBRATIONAL_ANALYSIS
  PRINT_LEVEL MEDIUM
&END GLOBAL
&FORCE_EVAL
  METHOD QS
  &DFT
    BASIS_SET_FILE_NAME BASIS_MOLOPT
    POTENTIAL_FILE_NAME GTH_POTENTIALS
    CHARGE 0
    MULTIPLICITY 1
&PRINT
   &MOMENTS
   &END
 &END    
&MGRID
      CUTOFF 300
      NGRIDS 5
      REL_CUTOFF 70
    &END MGRID
    &QS
      EPS_DEFAULT 1.0E-12
      WF_INTERPOLATION ASPC
    &END QS
   &SCF
      SCF_GUESS ATOMIC
 EPS_SCF 1.0E-7
 MAX_SCF 1000
 CHOLESKY INVERSE
 ADDED_MOS 50
      &DIAGONALIZATION
       ALGORITHM STANDARD
      &END DIAGONALIZATION
      &MIXING
       METHOD BROYDEN_MIXING
      ALPHA 0.4
      NBROYDEN 8
      &END MIXING
    &END SCF
    &XC
      &XC_FUNCTIONAL
        &PBE
        &END PBE
      &END XC_FUNCTIONAL
      &vdW_POTENTIAL
        DISPERSION_FUNCTIONAL PAIR_POTENTIAL
         &PAIR_POTENTIAL
            PARAMETER_FILE_NAME dftd3.dat
                TYPE DFTD3
                REFERENCE_FUNCTIONAL PBE
                R_CUTOFF 15.0
        &END PAIR_POTENTIAL
     &END vdW_POTENTIAL
    &END XC
    &POISSON
      PERIODIC xyz
POISSON_SOLVER WAVELET
&END POISSON
  &END DFT
  &SUBSYS
    &CELL
ABC 20.0000 20.0000 20.0000
ALPHA_BETA_GAMMA 90.0 90.0 90.0
      PERIODIC xyz
    &END CELL    
&COORD
O    -0.762288846626      0.000000000001     -1.462238785752
H    -0.762288846634      0.759337000000     -0.866195785751
H    -0.762288846634     -0.759337000000     -0.866195785753
&END COORD
    &KIND Mg
      BASIS_SET DZVP-MOLOPT-SR-GTH
      POTENTIAL GTH-PBE-q10
    &END KIND
    &KIND O
      BASIS_SET DZVP-MOLOPT-SR-GTH
      POTENTIAL GTH-PBE-q6
    &END KIND
&KIND Y
ELEMENT Y
BASIS_SET DZVP-MOLOPT-SR-GTH
POTENTIAL GTH-PBE-q11
&END KIND
&KIND C
ELEMENT C
BASIS_SET DZVP-MOLOPT-GTH
POTENTIAL GTH-PBE-q4
&END KIND
&KIND H
ELEMENT H
BASIS_SET DZVP-MOLOPT-GTH
POTENTIAL GTH-PBE-q1
&END KIND  
&END SUBSYS
&END FORCE_EVAL
&VIBRATIONAL_ANALYSIS
FULLY_PERIODIC  
INTENSITIES
        NPROC_REP 4
&END VIBRATIONAL_ANALYSIS
&MOTION
  &CONSTRAINT
    &FIXED_ATOMS
    &END FIXED_ATOMS
  &END CONSTRAINT
&GEO_OPT
  OPTIMIZER LBFGS
TYPE MINIMIZATION
MAX_DR 4.5E-4
MAX_ITER 500
&END GEO_OPT
  &END MOTION

[Frederick Stein]

Dear Naina,

Considering this information (https://gaussian.com/vib/), both codes should agree. If you say that the frequencies (or wavenumbers) are matching (with what error??), the force constants should match as well because they are directly related to each other (see the document). Considering your input, I have the following suggestions

- The cutoff could be too low. Secondary, double-check EPS_DEFAULT and EPS_SCF.

- If you actually have a gas phase system: Change the periodicity to NONE (in the cell definition and in the POISSON section) and remove the FULLY_PERIODIC keyword in the VIBRATIONAL_ANALYSIS section. Add the TOPOLOGY section with the CENTER_COORDINATES section (see https://manual.cp2k.org/trunk/CP2K_INPUT/FORCE_EVAL/SUBSYS/TOPOLOGY/CENTER_COORDINATES.html ).

Best,

Frederick

[Jürg Hutter]

Hi
Are you using a newer version of CP2K (2023.1 or later)? If not, please update your version.
If yes, please send an example input and output file in order to reproduce the problem.

regards
JH

________________________________________
From: cp...@googlegroups.com <cp...@googlegroups.com> on behalf of Naina Sethi <sethin...@gmail.com>
Sent: Saturday, August 23, 2025 4:15 PM
To: cp2k
Subject: [CP2K:21769] Mismatch between CP2K computed Force constant (a.u)and Gaussian computed force constant(mDyne/angstrom)

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[Naina Sethi]

I am using CP2K version 2024.3 and attempted to reproduce one of the sample calculations on the benzene molecule (https://www.cp2k.org/exercises:2015_ethz_mmm:infra_red). However, I am unable to obtain force constants similar to those that i get from gaussian. While the vibrational frequencies, IR intensities, and reduced masses agree perfectly with Gaussian results, the force constants differ significantly(specifically for lower frequency regime). This discrepancy is concerning because the magnitude of the force constants is critical for my work. Could you please advise whether this might be related to changes in the code since the example was prepared, or if I might be missing something in my setup?

I am attaching the Input and output files from CP2K and gaussian calculation. Kindly suggest. 

Thank you very much for your time and guidance.

Best regards,

[Naina Sethi]

Dear Prof. Hutter,

Is there any mistake in my input setup? Could you please help.