AMBER Archive (2007)

Subject: RE: AMBER: Failure to SCF convergence

From: Ross Walker (ross_at_rosswalker.co.uk)
Date: Sat Aug 11 2007 - 18:14:54 CDT

Hi Francesco.

> While carrying out a AM1 QM-MM md2 procedure (restarting from
> previous md1.out)
> I noticed casually from tail -f that after 1000 steps
> (precisely at step 291)
> SCF was not reached at the tolerances specified. Procedure
> continues with
> warning about energy reliability.

This is a function of the self consistent field procedure not being an exact
science. Sometimes, particularly if you have a 'weird' structure the
converger can get stuck and not get the energy or density difference below
the tolerances. If this only happens once or twice you probably don't need
to worry about it. You might want to write ntwx every step for the first 300
steps or so and take a look at the structure of step 291 and see if it looks
strange at all.

The issue with energies and forces is that the accuracy of the forces is a
function of how well the SCF gets converged. 10^-8 is typically needed for
energy conservation. However if the SCF procedure got close then the error
won't be big and is unlikely to affect your dynamics. Take a look at DeltaE
and DeltaP that it gives you for the last step of SCF, if it is of the order
of 10^-7 for deltaE and 10^-5 for DeltaP or so you are probably okay. Also
see what the smallest deltaE and deltaP are this tells you what was the
closest it got to converging.

You can up the number of SCF steps with the itrmax option in the qmmm
namelist but typically it won't help you for small systems that occasionally
show convergence issues.

Simple option is just to ignore it if deltaE is close to 10^-8 and it only
occurs once or twice.

> md1.out did not reveal problems in the final summary. Are
> such problems with
> SCF convergence reported in the final part of md.out?

No they are not, they are only printed to the output file as they happen. I
could potentially add a Warning at the end of an output file if convergence
issues were seen during a run. This might be useful information, I'll look
into how best to do it.

All the best
Ross

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|\oss Walker

| HPC Consultant and Staff Scientist |
| San Diego Supercomputer Center |
| Tel: +1 858 822 0854 | EMail:- ross_at_rosswalker.co.uk |
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