[CPMD-list] negative frequencies obtained after vibrational analysis

[Axel Kohlmeyer]

On Tue, 29 May 2007, Nagesh Babu Idupulapati wrote:

NBI> Dear all

dear nagesh,

NBI> I did geometry optimization for a particular system and then  
NBI> vibrational analysis to find out whether the optimized system is  
NBI> ground state or not.

could you comment on where you got your initial coordinates from
and whether you are looking at a crystal structure or an isolated
molecule?

NBI>  From the output file, I found more negative harmonic frequencies as  
NBI> shown below:
NBI> 
NBI> 
NBI> OUTPUT FILE FROM VIBRATIONAL ANALYSIS:
NBI> 
NBI> HARMONIC FREQUENCIES [cm**-1]:
NBI> 
NBI>       -72559.0607     -28053.3624      -1009.0848           .0012

are you certain that you restarted from the optimized coordinates?

NBI>             .0046           .0081           .0101           .0115

...

NBI> So my question is, the geometry optimization parameters I used in my  
NBI> input file were not sufficient to obtain the ground state?

if you have many degrees of freedom and start from a very symmetric
initial structure, the geometry optimizer may not find the minimum
that you are aiming at. you can try running a short MD and/or break 
symmetry with the RANDOMIZE keyword.

cheers,
   axel.



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Axel Kohlmeyer   akohlmey at cmm.chem.upenn.edu   http://www.cmm.upenn.edu
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