A number of things:
first:
Why are you using a cap and not a box/PME, etc ?
Second:
did you minimize with MM only first ?
A bad geometry can easily give you an unconverged QM
For the minimization, in general is not a good idea to use shake (ntc=2,
ntf=2). You should not use shake for minimization, and then turn it on
if you want.
Third:
After the MM minimization, try an AM1 or PM3 minimization to relax
further from your MM force field towards a more QM one.
Only after that try SCC-DFTB
Fourth.
Are you sure that it is worth your time to treat a FULL 98 atom residue QM ?
What would you gain ? Certainly, you can do MD but hot sure you can do
enough to sample correctly. If you want to sample a couple of torsions
or such, then ok, but you will need umbrella sampling or some other
method. Straight MD, or minimization, are unlikely to take you too far.
Francesco Pietra wrote:
> Now the 98-atoms solute is well inside TIP3PBOX cap (prepin from antechamber).
> After successful minimization with:
>
> Initial min for qmmm with dftb
> &cntrl
> imin=1, maxcyc=500, ncyc=200,
> cut=20.0, ntb=0, ntc=2, ntf=2,
> ivcap=0, fcap=10,
> ifqnt=1
> /
> &qmmm
> qmmask=':1',
> qmcharge=0,
> qmtheory=7,
> qmshake=0,
> qm_ewald=0
> /
>
> MD with:
>
> 300K constant temp for qmmd with dftb
> &cntrl
> imin=0, ntb=0
> cut=20.0, ntc=2, ntf=2,
> tempi=300.0, temp0=300.0,
> ntt=3, gamma_ln=1.0,
> nstlim=20000, dt=0.002,
> ivcap=0, fcap=10,
> ntpr=100, ntwx=100,ifqnt=1
> /
> &qmmm
> qmmask=':1',
> qmcharge=0,
> qmtheory=7,
> qmshake=0,
> qm_ewald=0
> /
>
> sander.MPI (mpirun -np 4) exited because:
>
> vlimit exceeded for step 2886; vmax = 26.4356
> SANDER BOMB in subroutine eglcao <qm2_dftb_eglcao.f> :
> SCC Convergence failure - MAXITER exceeded.
> Exiting
>
> Much obliged for suggestions what to do best. Thanks
>
> francesco pietra
>
>
>
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--
Dr. Adrian E. Roitberg
Associate Professor
Quantum Theory Project and Department of Chemistry
University of Florida PHONE 352 392-6972
P.O. Box 118435 FAX 352 392-8722
Gainesville, FL 32611-8435 Email adrian.qtp.ufl.edu
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Received on Sun Aug 26 2007 - 06:07:54 PDT
