# MM energy blow up after 2 steps although Geo opt works in QMMM

ghislain.m... at gmail.com ghislain.m... at gmail.com
Sat May 2 19:22:16 UTC 2015

```Hi all,

I have a iron(III) atom with its 6 coordinated water molecules (QM region)
surrounded by 2141 MM water molecules. I started with a geometry
optimization at the QM/MM level and that goes well (although only 1.5
charge is localised on the iron). I then start a MD run and things get
messy. The error is about one QM atom (the iron) being too close to a MM
atom. Then the MM energies go to the roof, I have NaN gradient and
evrything crashes. I don't understand how this works. First there are the
QM waters and other MM water molecules between the two "too close" atoms so
I don't think it's not a "real" geometry problem. I have tried to change
the size of the QM box, make the QM electrostatics periodic (initially set
to none periodic), try MT and multipole for the non periodic Psolver,
change the number of gaussian in GEEP_LIB, use 1 grid .. Nothing seems to
work. I have notice that the partial charge on the iron atom changes quite
a lot between 2 steps and when it's close to 0, the MM potential becomes
huge. I don't know why this is so however, and even if my iron goes
neutral, it should not fly into a MM water ...

1st step

MULLIKEN POPULATION ANALYSIS

#  Atom  Element  Kind  Atomic population (alpha,beta)   Net charge  Spin
moment
1     Fe       1          9.185313     5.416885     1.397803
3.768428
2     O        2          3.479264     3.199820    -0.679084
0.279444
3     O        2          3.454824     3.180460    -0.635284
0.274363
4     O        2          3.431022     3.274684    -0.705706
0.156338
5     O        2          3.463094     3.224418    -0.687511
0.238676
6     O        2          3.426629     3.272281    -0.698910
0.154349
7     O        2          3.412406     3.254903    -0.667308
0.157503
8     H        3          0.242603     0.240340     0.517057
0.002264
9     H        3          0.246114     0.251097     0.502789
-0.004983
10     H        3          0.251669     0.257771     0.490560
-0.006102
11     H        3          0.245008     0.247299     0.507693
-0.002291
12     H        3          0.266848     0.270467     0.462684
-0.003619
13     H        3          0.266236     0.272343     0.461420
-0.006107
14     H        3          0.258389     0.257856     0.483755
0.000534
15     H        3          0.268412     0.266475     0.465113
0.001937
16     H        3          0.271005     0.274766     0.454229
-0.003762
17     H        3          0.273854     0.272849     0.453298
0.001005
18     H        3          0.273299     0.277448     0.449253
-0.004148
19     H        3          0.284011     0.287840     0.428148
-0.003829
# Total charge and spin        33.000000    28.000000     3.000000
5.000000

[...] 3rd step

MULLIKEN POPULATION ANALYSIS

#  Atom  Element  Kind  Atomic population (alpha,beta)   Net charge  Spin
moment
1     Fe       1          9.000001     6.995986     0.004012
2.004015
2     O        2          3.564147     2.829655    -0.393802
0.734492
3     O        2          3.530082     2.891696    -0.421778
0.638385
4     O        2          3.477096     3.065067    -0.542163
0.412029
5     O        2          3.511836     2.968556    -0.480392
0.543280
6     O        2          3.497889     2.993893    -0.491782
0.503997
7     O        2          3.448838     3.094323    -0.543160
0.354515
8     H        3          0.219325     0.241090     0.539585
-0.021765
9     H        3          0.218180     0.242569     0.539251
-0.024390
10     H        3          0.232233     0.250945     0.516823
-0.018712
11     H        3          0.232325     0.250821     0.516853
-0.018496
12     H        3          0.250804     0.261371     0.487824
-0.010567
13     H        3          0.238978     0.258239     0.502783
-0.019260
14     H        3          0.255165     0.272122     0.472713
-0.016956
15     H        3          0.270316     0.281142     0.448542
-0.010826
16     H        3          0.247369     0.261604     0.491027
-0.014235
17     H        3          0.254925     0.269922     0.475153
-0.014996
18     H        3          0.267309     0.277233     0.455457
-0.009924
19     H        3          0.283180     0.293767     0.423053
-0.010586
# Total charge and spin        33.000000    28.000000     3.000000
5.000000

[...]

QMMM| Information on the QM/MM Electrostatic Potential:
QMMM| QM/MM Coupling computed collocating the Gaussian Potential Functions.
WARNING| Particles:     512      1 at distance [au]:     0.58442032 less
than:      0.95400000; increase EMAX_SPLINE.

********************************************************************************
* Value of r in Input =   0.584420 not in the spline range. Using =
0.901013 *
********************************************************************************
MM DIPOLE [BERRY PHASE](A.U.)|                  -9.832403   4.451024
19.766879
MM DIPOLE [BERRY PHASE](Debye)|                -24.991473  11.313373
50.242390
MM DIPOLE [BERRY PHASE] DERIVATIVE(A.U.)|        0.000464  -0.003523
-0.000552

ENERGY| Total FORCE_EVAL ( FIST ) energy (a.u.):
1560181.245483303442597

The most curious thing is that if I switch back from MD run to Geo_OPT run,
all is good, the charge on the iron stay stable and I can run it for as
long as I want. Then it would seem that my forcefield parameters are good,
otherwise the MM energy would not be stable there either.
I'm assuming this is is a silly mistake in my input but I can't work it out
and I was hoping one of you would be able to help me out.

Thank you,
G
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