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Re^5: How to get rid of Imaginary Frequencies.....


Dear Siddheshwar,

In the output of a calculation, the first few lines show what your input looked like. I copy-pasted those lines into a new input file together with the structure you performed your Hessian calculation on, changed the value of RUNTYP to GRADIENT, and then ran the job. Doing so, I saw a large gradient, which means that your geometry is not in a minimum. I could also have used RUNTYP=OPTIMIZE to see the same thing (as a geometry optimization always starts with a gradient calculation on your input geometry).

Of course I believe you when you say that you did a geometry optimization. :-) However, did you really perform the geometry optimization and the Hessian calculation with the same functional, basis set, etc.?

Also, and I would just like to ask this to rule out any possibilities (forgive me if it is a silly question); did you, after your geometry run completed, copy/paste the equilibrium geometry from the optimization output into the input file for the Hessian calculation?

Kind regards,

PS I should point out that a RUNTYP=GRADIENT run is never a substitute for a full Hessian calculation, but it can be used as an... 'initial check'. Typically, there is no need to do such a check, except in confusing cases such as this one. :-)

On Thu Feb 7 '13 12:54pm, Siddheshwar Chopra wrote
>Dear Thom,
>I am eager to know about your statement "When I start a geometry optimization form your structure, I find that the geometry you use in your Hessian calculation isn't a minimum geometry yet.".. Thomas how do you get an idea that it is a transition state BEFORE EVEN running hessian calculation? is there any method I can use to do know its local minimum nature before running a COSTLY hessian calc.?
>Yes, the file has imaginary freqs. but I would add here that I DID run RUNTYP=OPTIMIZE, before going for HESSIAN calc. And when  I completed the OPTIMIZATION job, I got the message, equilibrium geometry reached. What I knew is that I MUST run hessian calc. to know if it is a global minimum or not..
>However, I read somewhere that we can somehow find the global/local minimum nature by looking at the structure.. PLEASE HELP..

>On Thu Feb 7 '13 11:58am, Thomas wrote
>>Dear Siddheshwar,

>>When I start a geometry optimization form your structure, I find that the geometry you use in your Hessian calculation isn't a minimum geometry yet. You must always first optimize your structure with a RUNTYP=OPTIMIZE job before you can obtain a Hessian free of imaginary frequencies.

>>Note that you should always use the exact same settings during the optimization and Hessian calculation -- basis set, functional, and accuracy settings should be identical between the two. This is because different settings typically lead to different minima. A minimum calculated with pc-2 is different from a minimum calculated with 6-31G(d,p), and the Hessian will only be free of imaginary frequencies when the geometry really is in a minimum.

>>Hope this helps.
>>Kind regards,
>>On Thu Feb 7 '13 6:54am, Siddheshwar Chopra wrote
>>>Dear Thom,
>>>As asked by you, please find attached a sample file, which contains four imaginary frequencies..

>>>On Wed Feb 6 '13 11:36am, Thomas wrote
>>>>Dear Siddheshwar,

>>>>Could you please post the complete output of your Hessian calculation? This would allows us to see what kind of imaginary frequencies you're finding.
>>>>Kind regards,
>>>>On Wed Feb 6 '13 10:56am, Siddheshwar Chopra wrote
>>>>>Dear Sir,
>>>>>This in regard to my problems with getting rid of imaginary frequencies in the vibrational data. I have tried changing the basis sets, changed the ITOL, ICUT values... Please suggest me what should I do to ge trid of imaginary frequencies?? Currently I am trying out PC-2 basis set. A sample input commands are as follows:

>>>>> INPUT CARD> $SYSTEM MEMORY=12065831 aoints=dist TIMLIM=525600 KDIAG=1 $END                
>>>>> INPUT CARD> $CONTRL DFTTYP=B3LYP COORD=UNIQUE NZVAR=132 $END                              
>>>>> INPUT CARD> $CONTRL MAXIT=10000 MOLPLT=.t. PLTORB=.t. $END                                
>>>>> INPUT CARD>! $CONTRL EXETYP=CHECK $END                                                    
>>>>> INPUT CARD> $BASIS GBASIS=N31 NGAUSS=6 NDFUNC=1 NPFUNC=1 $END                              
>>>>> INPUT CARD> $STATPT opttol=10E-7 NSTEP=200 Method=GDIIS HSSEND=.TRUE. $END                
>>>>> INPUT CARD> $SMP HTTFIX=.f. $END                                                          
>>>>> INPUT CARD> $p2p p2p=.t. mxbuf=2048 dlb=.t. $END                                          
>>>>> INPUT CARD> $SCF dirscf=.t. fdiff= .f. diis=.t. $END                                      
>>>>> INPUT CARD> $DFT HFX(1)=0.15 $END                                                          
>>>>> INPUT CARD> $ZMAT DLC=.t. AUTO=.t. $END                                                    
>>>>> INPUT CARD> $FORCE NVIB=2 $END                          

>>>>>Please help me.

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