[CP2K-user] [CP2K:21772] Re: Mismatch between CP2K computed Force constant (a.u)and Gaussian computed force constant(mDyne/angstrom)

[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

Naina Sethi schrieb am Samstag, 23. August 2025 um 17:17:05 UTC+2:

> 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
>
> On Saturday, August 23, 2025 at 7:45:05 PM UTC+5:30 Naina Sethi wrote:
>
>> *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
>>
>

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