[CPMD-list] from phonons to spring constants

[Endres, Robert]

Hi all,
I tried to calculate the phonon frequencies
of a single, initially relaxed water molecule. The 
idea is to derive spring constants and to subsequently use 
Hook's law to calculate the forces on the three atoms 
for an arbitrary water structure. 

In addition to files HESSIAN and MOLVIB, the data below 
is the raw output from CPMD. First, does anyone know 
what "Projection (by PURGED routine)" means? Furthermore,
any ideas on how to obtain the vibrational modes
corresponding to the frequencies below? The deviations
from equilibrium for each mode times its sping constant 
would presumably give the negative force.

Any help is appreciated?
Regards,
   Robert


*********************************************************
 Raw second derivative matrix: No trans/rot elimination
  Harmonic frequencies in cm^-1:
      -736.9      -634.5      -574.4      -339.5
      -229.5        19.9      1517.4      4121.3
      4238.3
 Translations eliminated from second derivative matrix
  Harmonic frequencies in cm^-1:
      -735.6      -634.3      -574.4         0.0
         0.0         0.0      1517.4      4121.3
      4238.3
 Projection (by PURGED routine):
  Harmonic frequencies in cm^-1:
     -1002.3      -692.8      -574.4         0.0
         0.0         0.0      1520.7      4122.8
      4223.3