# [molpro-user] Problem regarding IRC with method QSDPATH

G Mahesh gmahesh at students.iiserpune.ac.in
Fri Aug 23 07:29:26 BST 2013

```Dear Ramon,

I thought, the problem arising in IRC was because of negative Hessian
eigenvalues but, now I realized that's not the case. Earlier I have done
transition state optimization with Hessian calculated by forward
differences and IRC with central differences so that, it was giving 4
negative Hessian values in the case of IRC.

Now I ran IRC with calculating Hessian using forward differences and  all
positive Hessian eigenvalues except one got too. Even now also surprisingly
giving the same error which I got earlier. Here, I also have posted the
echo of output having error.

***********Partial out put************

Geometry optimization using default procedure for command MULTI

Geometry written to block  1 of record 700

Generated   9 symmetrical displacements

Numerically approximating hessian using forward gradient differences

Task list generated. Total number of displacements:      9

5 tasks completed, CPU=1h 3m 15s Elapsed=1h 10m 13s

Numerical MULTI000 hessian completed. CPU-time:  6877.21 sec, Elapsed:
7645.49 sec

MULTI hessian saved to record  5300.2

Quadratic Steepest Descent -  Reaction Path Following using exact Hessian

Hessian eigenvalues:    -0.012734  0.000786  0.011948  0.018747  0.075415
0.141616  0.303108  1.646533  1.810536

IDIR= 1 requested but starting point is not a stationary point. Gradient
norm=  0.98D-04  Step length=  0.36D-01
? Error
? Not a stationary point
? The problem occurs in QSDPATH1
***********

What's wrong I am doing here....??

Thanks,

Mahi.

On Wed, Aug 21, 2013 at 9:18 PM, Ramon Hernandez <ramon at uaem.mx> wrote:

> Dear Mahi,
>
> I suggest you double-check that indeed your initial CASSCF calculation
> corresponds
> to a transition state, perhaps run a frequency calculation with tight
> parameters. Then
> when you run the IRC make sure you indeed converge to the same energy
> as the original
> CASSCF calculation. If you are sure about both these points then the
> problem might really
> have to do with a bug in IRC.
>
> Also if you run again the initial frequency calculation you can save
> that information and
> then read it directly in the next IRC job so you avoid calculating it
> twice.
>
> Cheers
>
> Ramon
>
> 2013/8/21, G Mahesh <gmahesh at students.iiserpune.ac.in>:
> > Dear all,
> >
> > I am familiar with MOLPRO from last two months. Since then I am trying to
> > find a transition state for the reaction of reactants and products having
> > multi reference character so, I am using CASSCF method for my problem.
> > After a long struggle, I have found the transition state with one
> imaginary
> > frequency (150 cm-1) and this normal mode having imaginary frequency
> seems
> > like reaction mode which will connect reactants to products.
> >
> > Now the problem is with IRC, here, I started IRC with the geometry which
> I
> > got from the transition state optimization with QSDPATH method and IDIR=1
> > then, after Hessian calculation, it's complaining that, the given
> structure
> > is not stationary with 4 negative Hessian eigenvalues. Does this means,
> my
> > initial structure is not real transition state structure.....??  Should I
> > have to try for the transition state structure again ..?? Any ideas to
> > solve the problem....?? Pls help me...!
> >
> > Below additional details may be required to understand the problem.
> >
> > We are using the MOLPRO version 2012.
> > Our system has 5 atoms. We are using 13 active orbitals and 15 active
> > electrons as my cas space and AVTZ as basis.
> >
> > Thanks,
> > Mahi
> >
>
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