Single Point calculation not converging

[krishnad]

Hi All,
    I am using Turbomole v6.3. I am trying to perform single point calculation for a 309 atom molecule using dispersion-corrected DFT (DFT-D3) and basis set def2-TZVP. After 30 iterations the .output file says "convergence criteria cannot be satisfied within 30 iterations !" and "ATTENTION : $energy will not be updated since this calculation did not converge (sob)" and "A POSSIBLE REASON MAY BE THAT ...
 - THE GEOMETRY IS FAR FROM BEING REASONABLE". Any suggestions as to what might be going  wrong? The molecule is energy minimized using molecular mechanics methods prior to submitting for SP calculation.

[antti_karttunen]

Hi,

Please check how the energy behaves during the last SCF iterations. If the energy seems to be converging smoothly, you could just increase the max. number of iterations. Edit the control file and increase $scfiterlimit to 100. If the energy is not convergning smoothly, there are many ways to improve the convergence. For example:
$scfdamp start=1.500 step=0.100 min=0.500
$scforbitalshift automatic=.2

Manual has more information on these keywords. In very difficult cases, you could also check the $fermi keyword for Fermi smearing.

Best,
Antti

[krishnad]

Thanks for the reply Antti. I will try your suggestions and get back with the results.

Cheers