Subject: Re: CCL:mn complexes

From: Achim Kless <[email-masked]>
Date: Wed, 30 Apr 1997 11:29:34 -0700 (PDT)

Dear Michael,

> I have a really bad problem and I cannot do anything about it.
> Here it is:
> I am trying to run ab-initio calculations on Mn(CO)3 and Cr(CO)3, using
> GAMESS-UK.
> I have tried the following methods (I also give if ti was succesful or
> not):
>
> 1) RHF/3-21G (3-21G on all atoms)
> for Mn(CO)3 this is successful, for Cr(CO)3 I get excessive number of
> iterations and an oscillation of the energy.
>
> 3) RHF with minimal basis ECP on all atoms for Mn(CO)3 and Cr(CO)3.
> This gives a ludicrously low energy (about -70a.u.), oscillations in the
> energy and excessive number of iterations
>
> 5) RHF, min. basis ECP and ECPDZ on Mn, 3-21G and 6-31G on C and O

try out the basis sets from A. Schaefer, C. Huber, and R. Ahlrichs. They
are
suitable especially for the middle-transition metals.
See: Fully Optimized Contracted Gaussian Basis Sets of Triple Zeta Valence
Quality for Atoms Li to Kr. A. Schaefer, C. Huber, and R. Ahlrichs;
J. Chem. Phys. 100, 5829 (1994).
Regarding ECPs check out the Pseudopotentials from M. Dolg, H. Stoll, H.
Preuss.
(http://www.theochem.uni-stuttgart.de)

best regards,

Achim

---------------------------
Dr. Achim Kless
UCLA,
Dept. Chemistry and Biochemistry

This was very helpful, I found these bais sets useful and also Dunning's
double-zeta basis sets.
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answer 2:

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From: Rolad Hertwig <[email-masked]>
Date: Wed, 30 Apr 1997 19:51:16 +0200

Micheal,

welcome to the world of transition metal quantum chemistry!

I will try to give you some hints, but in addition you should get
in touch with people ( I mean personally), who have experience in the
field, if your thesis advisor has not.

> iterations and an oscillation of the energy.

Oscillation is not unusual in organo-metallic complexes. Play around
with SCF accelerators, like damping, diis, and shifting of virtual
orbitals. There is no general recipe to avoid this. Also, change your
starting geometry. Avoid too small bond lengths. Exploit symmetry, and
specify electronic occupations explicitly as far as this is possible (I
have little experience with GAMESS).

> energy and excessive number of iterations

ECPs emulate the shielding of core electrons. Since these electrons are
not treated explicitly, they do not contribute to the total energy,
therefore you get small total energies. Do not compare total energies
calculated with ECPs with those resuting from all electron calcs.

> Does anyone have any ideas for how I can use ECPs (we are limited to
about
> 1.2GB) or anything else to get these calculations to run (we do not yet
> have DFT).

Okay, ECPs are not t h e solution to your problems as long as you do not
have to worry about relativistic effects (Not crucial for 3d TMs). If
you use them, be sure to use the basis set that comes along with them,
since it has been specifically designed for that certain ECP.
You do not necessarily need more than 1 GB of disk space, unless you are
going to use correlated post-HF methods. DFT would be suitable for your
case, since HF is rather inappropriate. However it is not te worst to
start with.

I hope I could give you some general clues, but it is not easy to do
this via email for the problems you have. I would really recommend, you
rip some money out of yor advisor's pockets and travel somewhere for a
couple of weaks to learn the trade.
Good luck!

Roland

-----------------------------------------------------------------------
Roland H. Hertwig
Institut fuer Organische Chemie, TU-Berlin

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answer 3:

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From: Tom Cundari <[email-masked]>
Date: Wed, 30 Apr 1997 11:57:30 -0600

Dear Michael,

Noticed you said something about an RHF calc.  What is the
multiplicity of Cr(CO)3 and Mn(CO)3 fragments you are trying
to converge on?

Are you using symmetry?

Another trick you can try with ECPs is to do a quick first calc on
the 2nd row analogue (Mo for Cr, Tc for Mn) and if this converges
use this wavefunction to start off a subsequent calculation of
the first row metal.  2nd row transition metals often behave
better in the SCF since their bonding is more covalent and hence
the ligand fields are typically larger.  At any rate, with ECPs
the number of electrons should be the same and the symmetry
props of the MOs should be similar.

Good luck.

Tom

Tom Cundari
Associate Professor
Department of Chemistry
University of Memphis   (under T in the ACS Grad Directory!)

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answer 5:
From: chemistry-request (SMTPMAIL.chemistr) at PROFGATE
Date: 4/30/97 9:57AM
To: Michelle Pietsch at BVL60PO
*To: C=us; A= ; P=Internet; DD.RFC-822=chemistry(a)www.ccl.net at
X.400
Subject: CCL:mn complexes
-------------------------------------------------------------------------------

Michael,

To force convergence, increase the value of LEVEL (keyword for GAMESS-UK).
You
may increase this value until you force convergence.  Once the wave
function is
converging, decrease the value for LEVEL little by little until it is at
its
default value.  Using a large value for LEVEL will force convergence but
it may
converge in the wrong state.  After convergence, print out your molecular
orbitals and make sure that the correct orbitals are occupied.  If the
correct
orbitals are not occupied, use the SWAP command to exchange occupied and
virtuals until the correct orbitals are occupied.  Always check to make
sure
that you have the correct orbitals occupied after using the SWAP command.
You
may want to examine the molecular orbitals before convergence and make any
SWAPs
at that time.

I have found this procedure to work well for systems containing Cr and
other
metals.  You should be able to use ECPs and any basis set.  However,
increasing
the number of valence functions will increase the difficulty of
convergence.

Best of Luck,

Mickey

answer 6:

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From: Roberto Bianco <[email-masked]>
Date: Wed, 30 Apr 1997 09:13:44 -0600 (MDT)

Hi Michael,
looking at the Metal-C distance of 1.8 angstrom you give below, my
guess is that the atomic centers are too far apart for the wavefunction
to converge.  I would try with initial, artificially shorter Metal-C
distances (say 1.2-1.4 A) to achieve convergence, and then let the
geometry relax (optimize) through small steps, such that the "memory"
of the converged wavefunction is not lost in the process.
Roberto
--
Roberto Bianco / Department of Chemistry & Biochemistry
University of Colorado / Campus Box 215 / Boulder, CO 80309 / USA
[email-masked] / phone +(303) 492-3504 / fax +(303) 492-5894
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answer 7: