I am calculation for optimization of TS of an aromatic molecule and a hydroxyl radical. In this DFT calculation, I am using the functional B3lyp/6-311+G(d,p) in the Gaussian16. The charge and multiplicity was ensured as 0, 2. But at the end it gives the following error message:
Error on total polarization charges = 0.05019
SCF Done: E(UM062X) = -323.942120576 A.U. after 401 cycles
NFock=400 Conv=0.36D-03 -V/T= 2.0041
= 0.0000 = 0.0000 = 0.5000 = 0.7612 S= 0.5056
= 0.000000000000E+00
SMD-CDS (non-electrostatic) energy (kcal/mol) = 2.33
(included in total energy above)
Annihilation of the first spin contaminant:
S**2 before annihilation 0.7612, after 0.7501
Convergence failure -- run terminated.
1) Did you specify in the runfile that you perform a restricted or unrestricted calculation? It may be quite well that only one of them converges.
2) DFT is a single-reference method, so you may be lucky and a radical calculation delivers a somewhat OK result, but there's absolutely no guarantee that happens, so DFT may simply be a bad method in your case.
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