Hi all
I am running a DFT calculations for heterocyclic organic molecule (contains S and N - 2 each atoms) with more aromatic rings and 104 total number of atoms.
Input line is B3LYP/6-311g+(d) scf=qc opt freq
But I am getting an error "CPHF failed to converge in LinEq1". Even i tried with "hf" it is the same. Can some one suggest me how to solve this error.
Thanks in advance
Chethan
Convergence issues in both SCF and atomic steps is always a complex problem, having its multiple sources. At first try to verify (and possibly refine) the initial geometry.
Apart from that check these topics - it may also be some specific technical issue:
https://groups.google.com/forum/#!topic/ccc-support-group/EX759vToQBA
http://iopenshell.usc.edu/forum/topic.php?id=2960
Good luck!
What is the reason of using sqc=qc ? Did you try optimizing the system without this option first and it failed? If not, try the conventional SCF first.
Other ideas:
- check if the geometry is reasonable
- integral(grid=ultrafine) is basically mandatory for any DFT, use it
- do the optimization first in the 6-311g(d) basis set (the + diffuse functions may sometimes cause convergence problems). If it finishes OK, use the final geometry and reoptimize with +. If it doesn't work, try with something even smaller (6-31g(d)) first
- when you do qc, try a tighter convergence (perhaps scf=(qc,conver=9)) as well as even higher precision of integrals (integral(grid=ultrafine,acc2e=12))
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