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

Question

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

Answer 1

It's not a descriptive error message, but this occurs when you reach the max number of iterations for the Eigenvector Following (EF) algorithm and still haven't converged a lambda. While Gaussian uses a different approach (the Berny Algorithm) for methods which have analytical gradients, CCSD(T) is one of the few methods for which these still aren't implemented, so it has to use EF to find minima/saddle points.

This page gives a mathematical description of the algorithm showing what lambda means in this context (note, the page is for QChem, but the underlying algorithm should be more or less the same in both programs).

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.

Also, if you haven't already, try to find your minima/saddle point with a lower level of theory first (e.g. DFT, MP2, CCSD) and use this as your starting geometry.