# CCSD(T) Transition State and Ground State Calculations in Gaussian09: “Unable to Determine Lambda”?

I’m doing CCSD(T) calculations (for both transition and ground state geometries) on a series of small molecules in Gaussian09. Several similar calculations terminated without any problems but one TS calculation and one ground state calculation gave me this error after about 20 minutes.

HESSIAN DOES NOT HAVE THE DESIRED LOCAL STRUCTURE TAKING P-RFO STEP SEARCHING FOR LAMDA THAT MAXIMIZES ALONG THE LOWEST MODE VALUE TAKEN LAMDA= 3.41177479 SEARCHING FOR LAMDA THAT MINIMIZES ALONG ALL OTHER MODES UNABLE TO DETERMINE LAMDA IN FmD114

Without these remaining calculations, several months’ worth of work is incomplete. I will appreciate any suggestions. I’ve searched a lot of chemistry forums, but nobody seems to know the solution.

Here’s my route section for the TS calculation:

#p opt=(ts,z-matrix,noeigentest) freq=noraman rccsd(t)/genecp nosymm maxdisk=15950MB

Route section for the ground state calculation:

#p opt=(z-matrix,noeigentest) freq=noraman rccsd(t)/genecp maxdisk=15950MB

• +1 Interesting question and on-topic. Just FYI you can use the backquotes  to do code highlighting. That is more standard than bolding. Also, it appears that your commands are more complex than a simple CCSD(T) calculation. You are doing TS optimization? That changes the possible errors quite a lot, but no mention in the text about that. – Cody Aldaz May 24 at 20:02
• I agree with Cody, that your question would have been easier for us to help with, if you told us you were doing a TST calculation. It turns out, the answer had nothing to do with CCSD. Please don't be shy to tell us exactly what you're doing! – Nike Dattani May 24 at 21:14
• Thank you. Actually, the CCSD calculation was for ground state geometry. Only the CCSD(T) calculation was for the transition state. I've probably mechanically included the TS keyword in the route section. My apologies. Anyway, it appears, the error occurs both for TS search and ground-state search. My systems are hypervalent hydrides of p-elements. – QuantumX May 24 at 23:54
• @QuantumX that change was my bad, I assumed you'd lost the TS in the second calculation. I'm surprised the CCSD runs through that part of Gaussian for a regular optimization, but I'm no expert on the optimization procedures and Gaussian has a tendency of surprising me haha. – Tyberius May 25 at 0:04
• Can you edit the main question with the most current best version of the question? Some of the details are in the comments. You don't need to say edit either as all of the versions are already saved. – Cody Aldaz May 26 at 16:33

There is probably a more in depth solution to your problem, and it would probably require a bit more knowledge about your system. However, for now you can try adding iop(1/6=N) to allow it to try more iterations (see here) There might be an iteration count somewhere in your output, but otherwise the default number of steps can't go higher than 40 regardless of system size, so maybe set N to something >40.
• Update: I've used iop(1/6=70). This time the calculation ran about 50% longer before terminating with the same error. – QuantumX May 25 at 8:51