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Thomas Pijper

thomaspijper@hotmail.com

Dear olgerdovich,

The problem of the SCF not converging is due to partial linear dependence in your basis. This is a very common problem when diffuse functions are employed. Partial linear dependence is indicated by the following lines in your output:

* * * WARNING * * * ------------------------------------------------------------------------------ THE OVERLAP MATRIX HAS 1 EIGENVALUES BELOW 1.5E-04. THE SMALLEST OF THESE IS 4.34786E-05. THIS INDICATES A PARTIAL LINEAR DEPENDENCE IN YOUR ATOMIC BASIS. TO OBTAIN SCF CONVERGENCE MAY REQUIRE MORE ACCURATE INTEGRAL EVALUATION (INTTYP=HONDO, ICUT=11, ITOL=30 IN $CONTRL), MORE ACCURATE DIRECT SCF FOCK MATRIX FORMATION (FDIFF=.FALSE. IN $SCF), OR CHANGING CONVERGERS (DIIS=.T. SOSCF=.F. IN $SCF). EIGENVALUES BELOW 1.0D-07 PROBABLY WON'T CONVERGE. EIGENVALUES BETWEEN 1.0D-07 AND 1.0D-06 MAY REQUIRE TIGHTENING OF -NCONV- DENSITY CONVERGENCE IN $SCF.

Please see the manual for tips on how to solve this problem.

With respect to your second question, the B3LYP functional contains terms describing electron correlation so this should be accounted for. Note though that DFT may not always be accurate, for example when considering long-range electron-electron interactions.

Hope this helps.

Kind regards,

Thom

On Fri Jan 9 '15 1:57am, olgerdovich wrote

------------------------------------------

>Dear colleagues,

>I'm a newbie in computational chemistry, but with some diligence I started learning it in relation to quite simple chemical tasks from the field of nitrocompounds and I've stumbled upon several problems. First of them is the subject of this post.

>When I ran geometry oprimization with hessian computing for 2-nitropropane, first at HF/6-31++G** and then at B3LYP/6-31++G** level (with starting geometry form HF-calculation), I found no problem, there were convergences to ground states.

>Next I ran geometry oprimization with hessian computing for 2-nitropropane radical anion at ROHF/6-31++G** and once again found no trouble. It was surprising to me that some vibrations were very intense in IR to the contrary to their analogues for neutral particle, but I assumed that it was rather reasonable taking into account concomitantly changed spin and charge. The resulting species was in ground state again.

>But when I started geometry oprimization (with hessian computing) for 2-nitropropane radical anion at B3LYP/6-31++G** level (with starting geometry form ROHF-calculation), run crashed on the first iteration cycle for SCF even when I indicate "maxit=99" (at higher values compilator returned error, it seemed like maxit should nor be larger then NSTEP=100 in $STATPT or just larger then 100): Energy changes started to oscillate and resulted in no convergence.

>Output-file is attached

>What may I adjust in my input to get result?

>Hoe can I take into account electron correlation for anion-radical like the above in general?

>Thank you in advance

*[ This message was edited on Fri Jan 9 '15 at 8:54pm by the author ]*

Fri Jan 9 '15 8:54pm

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