[molpro-user] ccsd(t) frequencies

[Li, Jun]

I am not sure if I fully understand your question, but in general one
has two possibilities:

(1) your geometry optimization locates a stationary point, which is not
a minimum.  Then you will really have some imaginary frequencies.

(2) your molecule is indeed optimized to its (local or global) minimum,
but the PES is so flat that you have very small REAL frequencies.  These
small frequencies are difficult to be calculated accurately and will
actually show as imaginary unless your geometry optimization reached
very tight convergence criteria (which then requires high accuracy in
the integrals and the SCF convergence).  

Jun Li

-----Original Message-----
From: owner-molpro-user at molpro.chem.cf.ac.uk
[mailto:owner-molpro-user at molpro.chem.cf.ac.uk] On Behalf Of Omololu
Sent: Monday, August 22, 2005 3:22 PM
To: molpro-user at molpro.net
Subject: [molpro-user] ccsd(t) frequencies

Dear All,

I did a ccsd(t) geometry optimization and located the minimum. However,
when i try to calculate the ccsd(t) frequencies, i get a couple of
imaginary frequencies in addition to the other ones.
Any suggestions would be appreciated.

o.
PS:
The relevant part of my input is:

 hf;                                                  
                          
 ccsd(t);                                             
                          
edimer=energy                                         
                         
 optg,rstep=0.0002,maxit=100,grad=3.5d-8;coord,3n
 frequencies
---

and the relevant part of my output:

 Imaginary Vibration  Wavenumber
        Nr             [1/cm]
        1               36.17
        2               18.21

   Low Vibration      Wavenumber
        Nr             [1/cm]
        1                7.36
        2               10.90
        3               33.85
        4               44.84

     Vibration        Wavenumber
        Nr             [1/cm]
        1             1645.08
        2             3810.40
        3             3919.50

 FREQUENCIES * CALCULATION OF NORMAL MODES FOR CCSD(T)

 Gradient norm at reference geometry: 0.37750D-04

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