AMBER Archive (2007)

Subject: Re: AMBER: SCC convergence failure in QM-MM DFTB

From: Gustavo Seabra (gustavo.seabra_at_gmail.com)
Date: Mon Aug 27 2007 - 12:20:37 CDT

Hi Francesco,

Working with QM/MM, and DFTB in special, is a bit trickier than just
classical MM. QM calculations are less tolerant for bad starting
geometries than MM, and you can easily get convergence errors. Here's
what I suggest:

1. Minimize the system with pure MM.
2. Do as you would in pure MM: Slowly relax the system to the final
temperature and pressure, using whatever ensembles you normally would,
all still using pure MM.
3. Only after your system is properly relaxed at the pure MM level you
turn on QM.

Also, notice that DFTB is a bit trickier to converge than the otherr
QM methods, so you may want to relax the system a bit with another QM
hamiltonian befor moving to DFTB, although you might be able to skip
this step. (I'd recommend PDDG/PM3 for that.)

Good luck,
Gustavo.

On 8/25/07, Francesco Pietra <chiendarret_at_yahoo.com> 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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