[AMBER] QM/MM dynamics: metal oxides

From: emanuele falbo <falbo.emanuele.gmail.com>
Date: Fri, 25 Jan 2019 17:03:49 +0000

Dear users,

I am running QM/MM dynamics on compounds (metal oxides, mainly the metal
atoms are M= Mo or W) solvated in water.

The solute molecule, let's say, is M^(n-) where n=3,10,27, so it can be a
highly charged system. Thus, counter ions needed to be added to neutralize
the total system. I do add Li+ ions with addions2 (atomic charges of solute
molecule is taken from Mulliken analysis, PBE optimization).

I do run the QM/MM dynamics at PM6/MM level of theory, because the PM6 is
the only built-in semi-empirical method working for these compounds in

When I run the dynamics for the system Li3M (all treated at PM6 level) it
works fine, and even if I get the following error, the dynamics continues
and at some point it seems running fine.

QMMM: Unable to achieve self consistency to the tolerances specified
QMMM: No convergence in SCF after 1500 steps.
QMMM: Job will continue with unconverged SCF. Warning energies
QMMM: and forces for this step will not be accurate.
QMMM: E = -0.3570E+07 DeltaE = 0.1048E-06 DeltaP = 0.5871E-09
QMMM: Smallest DeltaE = 0.1048E-06 DeltaP = 0.5871E-09 Step = 1500

However, when it comes to run, for example, the Li15M-where only the
M^(15-) is modelled at PM6 level and the remainder, included 15Li+, is
treated at MM-it gives the above error, and fails.

Of course, this makes me think that there is a problem with highly charged
systems, however, I am not sure how to get it work.
Therefore, I wonder if you have any idea about how to sort this issue out ?

Best Wishes,

*Emanuele Falbo*
PhD student
Penfold group
School of Natural and Environmental Sciences
Bedson Building, Newcastle University
Newcastle upon Tyne, NE1 7RU
w: http://tompenfoldresearch.weebly.com/
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Received on Fri Jan 25 2019 - 09:30:01 PST
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