[molpro-user] Problem regarding IRC with method QSDPATH

[Ramon Hernandez]

Dear Mahi,

I see two problems with your calculations:

a) The estimate of frequencies with central differences should be more
reliable and
 this calculation shows four negative hessian eigenvalues so doing an
IRC at this
 geometry is not correct. Look at the manual page for frequency calculations.

b) The program is complaining about the gradient value it seems it is not small
 enough to be considered converged for a stationary point.

I think first you have to make sure that you have really found a
transition state
with only one imaginary frequency doing the calculation with central
differences.

Cheers

Ramon

2013/8/23, G Mahesh <gmahesh at students.iiserpune.ac.in>:
> 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
>> >
>>
>