[CPMD-list] Very Large Negative Frequencies

[Ari P Seitsonen]

Dear Rad Balu,

  Did you specify RESTART GEOMETRY as well? Have you checked that your 
optimised geometry was indeed okay, and that it was indeed used (by 
checking the output of the corresponding calculation) when calculating the 
frequencies? Did you have the corresponding, optimised cell? I'd be 
somewhat careful taking the geometry from a calculation with 'TSDC', 
unless you checked later that the cell is indeed properly optimised (for 
example with respect to Pulay stresses).

    Greetings from Paris,

       apsi

-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-
 Ari Paavo Seitsonen / Ari.P.Seitsonen at iki.fi / http://www.iki.fi/~apsi/
 Tel +41 1 635 44 97 / Fax +41 1 635 68 38 / GSM +41 79 719 09 35      
 Anschrift: Physikalisch Chemisches Institut (PCI), Universität Zürich (UniZh)
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 Address:   Schweiz / Svizzera / Suisse / Svizra / Switzerland

On Tue, 1 Feb 2005, Rad Balu wrote:

> 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