I am doing a CCSD(T) geometry optimization on a series of molecules. One of the geometry optimizations took about 2 weeks even though an analogous molecule with the same number of basis functions took less then 2 days to converge to a minimum. After 30 iterations, the calculation aborted with the 'error' message about exceeding the maximum number of iterations. I checked energies at different geometry optimization steps and noticed that the optimization had taken the molecule further from the minimal energy value so far. Furthermore, the bondlengths are more reasonable for the optimization step corresponding to the minimal energy value so far: the molecule should be symmetric (and it is at the 4th optimization step) but it's becoming a bit too asymmetric as the optimization progresses. Also, the energy seems to only change by a miniscule amount in the last iterations. I used the optimized geometry obtained from a CCSD calculation as the initial guess. My route section: #p opt=z-matrix freq=noraman rccsd(t)/genecp maxdisk=15950MB
A mixed aug-cc-pVTZ-PP/aug-cc-pVTZ basis set was used in both CCSD and CCSD(T) calculatons.
Now, I'm not sure that simply increasing the number of iterations will help... Any advice would be greatly appreciated!
Update: I tried setting MaxStep to 5. It appeared to help at first because I reached a point where 3 out of 4 convergence criteria were satisfied. Even the remaining criterion was very close to the threshold value. The first calculation (with N=30) only produced some points with 2 satisfied convergence criteria. But then the calculation with MaxStep=5 started deviating as well. So I used this point with 3 satisfied convergence criteria as the initial guess for the next calculation (setting Maxstep to 1). A similar thing happened here. Here are the respective output fragments:
`Maximum Force 0.000117 0.000450 YES RMS Force 0.000060 0.000300 YES Maximum Displacement 0.001992 0.001800 NO RMS Displacement 0.001109 0.001200 YES`
`Maximum Force 0.000116 0.000450 YES RMS Force 0.000060 0.000300 YES Maximum Displacement 0.001911 0.001800 NO RMS Displacement 0.001007 0.001200 YES`