[CPMD-list] Very Large Negative Frequencies

[Rad Balu]

Dear All,

I am trying to do a vibrational analysis on a system (around 50 atoms) using CPMD version 3.9. I am getting large negative
frequencies as part of the output as shown here:
HARMONIC FREQUENCIES [cm**-1]:

     -23281.9544     -23272.5876     -22483.4345     -22463.8015
      -9610.4496      -9576.3246      -6998.5338      -6654.0035
      -6359.3503      -6311.7344      -4500.7134      -4161.7275
      -3625.4781      -3623.1194      -2297.5825      -2046.2618
      -1957.2234      -1920.9401      -1794.1146      -1721.5709


I did optimize the geometry before the vibrational analysis and a portion of the input is shown here:
&CPMD
 OPTIMIZE GEOMETRY
 TSDC
 LBFGS
 LANCZOS DIAGONALIZATION
 LANCZOS PARAMETERS
 1 6 10 1.D-8
 SPLINE RANGE
 6.0
 SPLINE POINTS
 3000
 RESTLINE
 2
 STRUCTURE BONDS ANGLES DIHEDRALS
 CONVERGENCE ADAPT
 0.000001
 RESTART LATEST

I tried to calculate the frequencies using PBC (w/o PBC the frequencies are normal) and here is my input for the run

&CPMD
 VIBRATIONAL ANALYSIS
 LBFGS
 HESSIAN DISCO
 SPLINE RANGE
 6.0
 SPLINE POINTS
 3000
 RESTLINE
 2
 STRUCTURE BONDS ANGLES DIHEDRALS
 CONVERGENCE ADAPT
 0.000001
 BIG MEMORY
 RESTART LATEST

I would appreciate if somebody can provide some suggestions to improve the results. Please let me know if you need more information
on how I set up the input.

Thanks
Rad Balu
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