Possible bug in reported force constants after vibrational analysis

Matthias Krack matthia... at psi.ch
Sun May 25 21:47:34 UTC 2014

Hi Leopold,

I looked at it again and I think now I was mistaken. You are right, the 
formulas for frequency and force constant are not consistent.


On Sunday, May 25, 2014 11:33:06 PM UTC+2, Leopold Talirz wrote:
> Dear Matthias,
> thank you very much for your quick reply.
> I am really not well familiar with normal mode analysis (nor its 
> implementation), so please tell me if I am talking nonsense, but I thought 
> the eigenvalues of the Hessian were proportional to konst instead of 
> SQRT(konst)?
> At least with the naive definition of a force constant that I am familiar 
> with, i.e. you have a potential like U=1/2 * konst * x^2
> Or, in other words, could you please direct me to a reference/book that I 
> should read to understand how the force constants are defined in CP2K?
> Thank you very much!
> Best,
> Leopold
> On Sunday, May 25, 2014 12:38:17 PM UTC+2, Matthias Krack wrote:
>> Hi Leopold,
>> I think the code is correct, since the eigenvalues epsilon of the Hessian 
>> returned in H_eigval2 after its diagonalisation are equivalent to 
>> SQRT(konst/rmass). The frequencies, however, are proportional to 
>> SQRT(epsilon), i.e. SQRT(H_eigval2).
>> Matthias
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