What syntax error was printed? Could you please put here the final part of your output? Did you run a TS calculation before at the DFT level?

Dear Juliana,

Thank you for your answer. Before running the IRCMax, I performed DFT calculations at B3LYP/6-3-11G(d,p) level of theory to find the saddle point. My input file for IRCMax is:

--------------------------------------------------------------------------------------

....

%chk=IRCMAx

#p IRCmax(CBS-QB3:B3LYP/6-311G(d,p),Zero)

IRCMax C2H5OH C2H4 + H2O CBS-QB3:B3LYP/6-311G(d,p)

0 1

"Geometry of saddle point from previous DFT calculations"

----------------------------------------------------------------------------------------

When I am trying to run the job I am immediately obtaining an error message :

"

-----------------------------------------

#p IRCMax(CBS-QB3:B3LYP/6-311G(d,p),Zero)

-----------------------------------------

QPERR --- A SYNTAX ERROR WAS DETECTED IN THE INPUT LINE.

#p IRCMax(CBS-QB3:B3LYP/6-311G(d,p),Zero

'

Last state="IRC2"

"

Additionally, I noticed that the above problem occurs only with CBS and G1-G4 methods. Is there a way to somehow use them with IRCMax ?

Hi Daniel,

Do you mind to send (or attach here) your output file? My e-mail is [email protected]

Usually the prime indicates where the syntax error is, and it seems that is over the #p which is strange. Maybe you can try to put the “Zero” optin before the method, right after the parenthesis like this:

IRCMax(Zero,CBS-QB3:B3LYP/6-311G(d,p))

'

Dear Juliana,

The prime in my log file indicates a problem with CBS-QB3, however, when I pasted it here, the prime was shifted backward due to formatting changes. Can you tell me if you can run IRCMax with CBS-QB3 and B3LYP without any errors? Changing the position of a "Zero" option or removing it completely also don't help.

CBS-QB3 is a composite approach, isn't it? I would suspect that this type of approach has to be used separately, never in combination with other methods (e.g. B3LYP). The same applied to Gaussian-# methods as they are also composite approaches.

it wouldn't make sense to use the CBS-QB3 keyword in an IRCmax job even if you don't get any error message, because it contains an initial optimization step to a minimum so it'll defeat the purpose of IRCmax, which is to obtain single-point energies along the IRC.

If you must use IRCmax with CBS-QB3, you have to run it "manually", so to speak, by doing single-points for every geometry along your IRC. A compound job like below would do:

%chk=h2.chk

# CCSD(T)/6-31+G(d')

--Link1--

%chk=h2.chk

# Geom=AllCheck Guess=Read MP4SDQ/CBSB4

--Link1--

%chk=h2.chk

# Geom=AllCheck Guess=Read MP2/CBSB3 CBSExtrap=(NMin=10,MinPop)

then use the Excel worksheet to get the CBS-QB3 energies:

http://www.chem.ucla.edu/~mccarren/CBSQB3_worksheet/

I wish that these composite methods would have a "single-point-only" option but there isn't.

Dear Juliana, Jose and Bun,

Thank you very much for your responses. I think that now I will be able to solve my problem. I have recently read a paper where the authors stated that they used IRCMax with CBS-QB3 method which is a composite approach that was correctly noticed by Jose. I thought then, that there maybe is some way to combine these two methods together. Thanks to Bun Chan I already know that it doesn't make sense and it has to be done separately.

Once again, thank you very much for help,

Hi Daniel Nurkowski

Could you please share the reference details where CBS-QB3 was used to do IRCMax.

Dear Daniel:

I faced this problem six years ago because I wanted to get IRCMax(CBS-QB3) for WKB approach. As far as I know, we cannot run this calculation automatically using Gaussian. Have you asked Gaussian's people this question? I have not.

For my case, I have to run this calculation manually, first run IRC with B3LYP, and then run single-point energy calculation with CBS-QB3.

For each geometry along the reaction path, you can run a single point energy calculation with CBS-QB3 as below:

1) #B3LYP 6-311G** freq

..

..

..

2) #CBS-QB3(StartMP2) geom=check guess=read

Do not hesitate to contact me: [email protected]

Lam