[CP2K-user] [CP2K:21771] Re: Mismatch between CP2K computed Force constant (a.u)and Gaussian computed force constant(mDyne/angstrom)

[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

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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