 ## Firefly and PC GAMESS-related discussion club

Learn how to ask questions correctly  Re: Conical Intersection Properties

Alex Granovsky
gran@classic.chem.msu.su

Dear Dobromir,

I'm sorry for huge delay with my reply.

You can obtain necessary information examining the output from the last step of CI optimization with conic code.

Here is a sample for CI in allene molecule optimized using conic code:

```
Norm(a2)     =    0.0591802683    Norm(b2)     =    0.0041383997
Norm(a1)     =    0.0591118891    Norm(b1)     =    0.0041244390

Alpha(a2,b2) =    0.9975690170    Alpha(a1,b1) =    0.9940447540
Alpha(a1,a2) =    0.9999372915    Alpha(b1,b2) =    0.9868473636
Alpha(a1,b2) =    0.9982753908    Alpha(a2,b1) =    0.9949129342

Lambda 1     =    0.5749404178    Lambda 2     =    0.5745876057
Lambda sc    =    0.5745891827    Lambda       =    0.5745891827

DELTA ENERGY IS         0.000099939393

```

To interpret the output, one needs take in mind that:

• a2 and a1 denote gradients of the first electronic state at current and previous geometries correspondingly.
• Similarly, b2 and b1 denote gradients of the second electronic state at current and previous geometries
• Norm(a2) denotes a norm of vector a2 etc...
• Alpha(a2,b2) denotes a cosine of angle formed by vectors a2 and b2 etc...
• Various Lambdas are approximations to Lagrange multiplier. The best approximation is denoted simply as Lambda.

The most important information one can extract from these numbers are
the norms of gradients at the MECI and the angle between gradients.

From the fragment of the output above, you can see that gradient
of the second state is small and hence we are close to an extremum
on its PES. Hence it is likely that MECI is peaked. However this may
require additional investigation e.g. an attempt of optimization
of a geometry of the second electronic state using the same state-
averaging.

On the other side, consider situation when both gradients are
large and Ācollinear i.e. with Alpha(a2,b2) approaching unity.
This would correspond to a sloped MECI.

> What is the easiest way to find the g and h vectors, since the
> algorithm does not use them ?

The g vector is simply the difference of gradients for two states.

It can be computed using runtyp=gradient with both istate and jstate
set. There will be three gradients in the output, namely (g1 - g2)/2,
g1 and g2 (in that order). Here, g1 is gradient for the first state
and g2 is gradient for the second state.

It is currently impossible to find h vector with Firefly.

>There is a \$HESS group in the PUNCH file, at the end of the calculation, can I learn from it ?

No, you cannot learn from it. It is very approximate.

>Also if I try vibrational analysis, would the correct way after state averaged CI run be:
>removing \$mcaver \$conic, and \$track, along with method=conic in \$statpt, and keeping the rest of the input the same, while adding the vibration specific input ?
>Should I use \$CONTRL NUMDER=.T. \$END ?
>That would keep things at nstate=2 istate=1 type of calculation, I guess.

It is not very good idea to perform vibrational analysis at MECI.
It is better to use some point near MECI. You need to remove \$conic
group and method=conic keeping the rest intact while adding
the vibration specific input. You need not to use numder.

>Or can I just run a single point with the exact same input as that from the CI search at the found geometry, but also adding hssend=1 at \$statpt. The CI search does switch between algorithms at some point, will the previous algorithm being active in a case like this, and change my results significantly ?

No you cannot use hssend=1

Kind regards,
Alex Granovsky

On Thu Apr 16 '15 7:32pm, Dobromir Antonov Kalchevski wrote
-----------------------------------------------------------
>Hello,

>I successfully found a conical intersection with your program. Now I am trying to figure out how to characterize it - is it sloped, peaked or seamed ? What is the easiest way to find the g and h vectors, since the algorithm does not use them ? Is it possible, if the CI is practically at the peak of the ground state surface, yet not completely (let's say 10 degrees deviation from a perfect middle geometry for the ground state TS, on the reaction path coordinate (my TS is product-similar, not reagent-similar, meaning may be 20 degrees in the wrong direction)), to still go forward through the saddle point and enter the other product. Sometimes a peaked CI has more than one TS around it and more than one product, I'm just trying to figure out what is the chance that I'm getting product from the CI.

>There is a \$HESS group in the PUNCH file, at the end of the calculation, can I learn from it ?

>Also if I try vibrational analysis, would the correct way after state averaged CI run be:
>removing \$mcaver \$conic, and \$track, along with method=conic in \$statpt, and keeping the rest of the input the same, while adding the vibration specific input ?
>Should I use \$CONTRL NUMDER=.T. \$END ?
>That would keep things at nstate=2 istate=1 type of calculation, I guess.

>Or can I just run a single point with the exact same input as that from the CI search at the found geometry, but also adding hssend=1 at \$statpt. The CI search does switch between algorithms at some point, will the previous algorithm being active in a case like this, and change my results significantly ?

>Thank you in advance, and Best regards,
>Dobromir A Kalchevski   Mon Apr 27 '15 1:43am   This message read 592 times