Hello,
I'm looking to set up a simulation system that contains two charged
molecules separated by some distance (20 Angstroms). I have solvated the
system and tried to minimize using DFTB but I get a convergence error at
every step. I have created some test systems, one being a methane-methane
system and found that the distance isn't the issue (as the methane-methane
had no problems). I believe it has something to do with the charge
separation. I've tried having a separation of charges treated quantum
mechanically (methanol anion and methane cation) at distances ranging from
5 A to 20 A and still see the same issue.
Here is the specific error:
--------------------------------------------------------------------------------
274 4. RESULTS
275
--------------------------------------------------------------------------------
276
277
278 QMMM SCC-DFTB: !!!! ============= WARNING ============= !!!!
279 QMMM SCC-DFTB: Convergence could not be achieved in this step.
280 QMMM SCC-DFTB: The calculation will continue, but energies and
281 QMMM SCC-DFTB: forces for this step will not be accurate.
282
283
284 NSTEP ENERGY RMS GMAX NAME
NUMBER
285 1 -7.7599E+04 1.5387E+01 8.7924E+01 C
1
286
287 BOND = 0.0000 ANGLE = 0.0000 DIHED =
0.0000
288 VDWAALS = 9091.9499 EEL = -86664.7073 HBOND =
0.0000
289 1-4 VDW = 0.0000 1-4 EEL = 0.0000 RESTRAINT =
0.0000
290 DFTBESCF= -25.9372
291
292 QMMM SCC-DFTB: !!!! ============= WARNING ============= !!!!
293 QMMM SCC-DFTB: Convergence could not be achieved in this step.
294 QMMM SCC-DFTB: The calculation will continue, but energies and
295 QMMM SCC-DFTB: forces for this step will not be accurate.
I've played around with the max number of iterations, convergence criteria
etc. I also tried other levels of QM theory in Amber and I get the same
result each time.
I am able to perform single point energy calculations on these systems just
fine in Orca (I tried it just to see).
Am I doing anything obviously wrong here? How should I be treating this
system?
My starting topology and coordinate files are fine. I prepared the QM
region with dummy charges from antechamber and solvated in leap.
Here is my input for the minimization. I should also mention that even
when I minimize treating the system with MM and then try to equilibrate I
get the same error, so this issue is not limited to minimizations.
Restrained minimization without SHAKE
&cntrl
imin = 1,
irest = 0,
ntx = 1,
ntmin = 1,
maxcyc = 1000,
ncyc = 500,
ntr = 1,
restraint_wt = 1.0,
restraintmask = ':1-2',
ntb = 1,
ntf = 1,
ntc = 1,
cut = 10.0,
ntpr = 1,
ntxo = 2,
ntwr = 1,
ioutfm = 1,
ntwx = 1,
iwrap = 1,
ifqnt = 1
&end
&qmmm
qmmask = ':1-2',
qmcharge = 0,
qm_theory = 'dftb'
qmshake = 1,
qm_ewald = 1,
qm_pme = 1,
scfconv = 1.0d-9,
tight_p_conv = 0
itrmax = 1000
&end
My overall system charge is zero.
Thanks,
AB
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Received on Tue Jun 04 2019 - 07:30:03 PDT
