CCL:G: MOPAC - Problem with Iron complex and gradients



Hi, I'm very sorry for my delay (I was having connection issues).
I'll give it a try and share my results.

Thank you.

2014-09-17 10:26 GMT-03:00 Henrique Castro Silva Junior henriquecsj|,|gmail.com <owner-chemistry#ccl.net>:

Sent to CCL by: "Henrique Castro Silva Junior" [henriquecsj**http://gmail.com" target="_blank">gmail.com]
Hi, I am an undergraduate in Chemistry from Brazil, writing my thesis as
a requirement to receive my graduation. Im working in thermodynamic (and
MO) descriptions that takes place in [Fe(OH2)6]2+ --> [Fe(OH2)6]3+,
mostly using computational chemistry softwares.
I have decided to use MOPAC2012 and Gabedit (as a GUI), but Im having
problems in getting realistic values. Here is my input:

* ===============================
* Input file for Mopac
* ===============================
PM7 THERMO ROT=1 EPS=78.39 RSOLV=1.3 UHF CHARGE=2 Quintet BONDS AUX
Mopac file generated by Gabedit
Fe -0.1136 1 -0.9091 1 0.0000 1
OÂ -0.1136Â 1 0.9109Â 1 0.0000Â 1
HÂ 0.5055Â 1 1.2122Â 1 -0.7127Â 1
OÂ 1.7064Â 1 -0.9091Â 1 0.0000Â 1
HÂ 2.0628Â 1 -1.3124Â 1 0.8307Â 1
OÂ -0.1136Â 1 -0.9091Â 1 -1.8200Â 1
HÂ -0.4817Â 1 -1.7682Â 1 -2.1477Â 1
OÂ -0.1136Â 1 -2.7291Â 1 0.0000Â 1
HÂ -0.8413Â 1 -3.0457Â 1 0.5936Â 1
OÂ -1.9336Â 1 -0.9091Â 1 0.0000Â 1
HÂ -2.2569Â 1 -1.5900Â 1 -0.6429Â 1
OÂ -0.1136Â 1 -0.9091Â 1 1.8200Â 1
HÂ -1.0440Â 1 -0.9414Â 1 2.1566Â 1
HÂ -2.2613Â 1 -0.0487Â 1 -0.3649Â 1
HÂ 0.7122Â 1 -3.0886Â 1 0.4108Â 1
HÂ -0.7542Â 1 -0.2257Â 1 -2.1427Â 1
HÂ 2.0192Â 1 -1.5032Â 1 -0.7289Â 1
HÂ 0.3089Â 1 -1.7285Â 1 2.1807Â 1
HÂ 0.3233Â 1 1.2180Â 1 0.8341Â 1

The result is this:

GRADIENT NORM = 298.62520
Â** GRADIENT IS TOO LARGE TO ALLOW FORCE MATRIX TO BE CALCULATED,
(LIMIT=10) **
ÂEITHER ADD 'LET' OR REDUCE GRADIENT
ÂUSING 'TS' OR OTHER GEOMETRY OPTIMIZER

If I add the keyword LET the calculation goes on, returning this:
     HEAT OF FORMATION =  Â-39.019942 KCALS/MOLE

With bond distance of ~1,82 A (much less than what is expected, right?).
Then, I tried to perform an optimization first, to reduce the gradient
and do not use LET:
PM7 XYZ EPS=78.39 RSOLV=1.3 UHF CHARGE=2 Quintet BONDS AUX

The result is a fragmentation, [Fe(OH)6] + 6H and the same result (the
fragmentation) goes with Gaussian09.
Any help will be very much appreciated and Im sorry if Im making some
obvious and stupid mistake here, but most of my knowledge on
computational chemistry Ive learned by myself.



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--
Henrique C. S. Junior
QuÃmica Industrial - UFRRJ
Centro de Processamento de Dados - PMP