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- It doesn't converge:
- What doesn't converge? The optimization?
The SCF? What?
- Optimization doesn't converge:
- Common solutions:
(1) Your guess geometry is so far off, the optimizer gets
hopelessly confused; (2) Your guess Hessian is so far off, the optimizer
gets hopelessly confused; (3) Why didn't you look at your outputs
to realize the energy you're optimizing isn't even right anyway? (For
example, it doesn't even converge the SCF or to the right SCF solution).
Of course it won't optimize!
(4) It's just a floppy molecule and needs some extra steps to get there.
This is the case if it looks like (based on your gradients and energies
at each step) you're making progress towards a minimum.
- Optimization converges to structure with imaginary frequencies:
- The following comments apply if you are trying to get a local minimum
but wind up with a saddle point.
Usually this happens when you start with a high symmetry structure.
Optimizers assume you know what you're doing, and if you give them
a C2v molecule, they will try to keep it C2v, even if there
is no true C2v local minimum, but only a saddle point.
have very weak imaginaries (say, less than 50i), it might just
be that you need to converge tighter (Q-Chem has a sleazy tolerance,
try tightening GEOM_OPT_TOL_GRADIENT below 300). This latter
problem happens frequently with larger and floppy molecules.
- Can't reproduce another program's energy:
- Check in the following
order: (1) Check the nuclear repulsion energy; this makes sure you
have the same geometry. Note, Q-Chem can be off in about the 4th digit
after the decimal, but this magically doesn't ever seem to matter...
(2) Check the SCF energy; this makes sure you have the same geometry
and basis set (and clears up any pure angular momentum vs.
Cartesian function confusion -- see below -- this mistake is
usually worth a couple of millihartrees or so). (3) Check that both
calculations freeze the same core/virtuals (freezing/not freezing core will
make a big difference in the total energy; the virtual part would
only make a small change on the millihartree or sub-millihartree
scale). (4) For MCSCF, you have to make sure the active space is really
the same (symmetry issues could make different active spaces even if
they have the same number of active orbitals).
- The SCF energy is all wrong:
- First, make sure you have the right
geometry (check the nuclear repulsion energy). Next, check the
basis set (remember pure angular momentum vs. Cartesians, see below).
Next, see if it looks like the right state (e.g., spin multiplicity,
charge). Next, see if it might have landed on the wrong SCF solution
(try changing the initial guess, or the convergence options like
DIIS, or try to check the SCF stability). Note that ACES II (and sometimes
other programs) can be very bad about guessing the wrong initial
orbital occupations. If this happens, give the program the orbital
occupations yourself, if possible (they can't really be specified
in Q-Chem). MOLCAS will guess the occupations wrong more than 90% of
the time. ACES II and PSI3 are usually right for closed-shell but often wrong
- It complains about not being able to read/write from/to the disk:
- It is probably having trouble reading/writing from/to the disk. Is the disk
full? Do you have the scratch files set up right? Are the files
really there? Is it trying to write a file that goes beyond some
operating system limit (e.g., 2GB, 4GB) or a limit on the queue you
- The frequencies (or ZPVE's) look wrong:
- Are you computing frequencies numerically by differences of gradients
computed at displaced geometries? If so, maybe you landed on the
wrong Hartree-Fock solution at one of the displaced geometries.
Next: Basis Sets