gaussian基本概念和用法(11)

------------------------------------------------------------ The Matt

Mr. Shan, when I have gotten odd convergence calls like this, the first thing I try is SCF=QC. This uses a quadratic converger for SCF that is usually more reliable. But, it is more expensive, so the job will take a bit longer. Try that and see, it usually works for me. Also, remember, if you have diffuse functions + DFT, it's always a good idea to use SCF=Tight as well. For a little more look at \http://www.gaussian.com/00000432.htm

Hope this helps, Matt Thompson Hope this helps Laurence Cuffe -------------------------------------------------------------- Ohyun Kwon

1) use looser convergence for scf at first step then read guess from checkpoint file. Use scf(conver=4,maxcyc=512) first or combine with vshift=400 as scf(conver=4,vshift=400,maxcyc=512). Then you would get scf convergence, and then use this guess for your new scf calculation.

2) To make sure everything is clear, use p at # section. Finally, use as follows. # b3lyp/gen Pseudo=Read

# scf(conver=4,vshift=400,maxcyc=512) . --link1--

# # b3lyp/gen opt(modredundant,maxcycle=200) Pseudo=Read # guess=read scf(vshift=400,maxcyc=512) . .

-------------------------------------------------------------- John Bushnell

For difficult convergence cases, it is nice to see the SCF convergence cycle-by-cycle. You can see this by putting a \becomes: #p b3lyp/gen opt(modredundant,maxcycle=200) Pseudo=Read

I ALWAYS do it this way. Now you can see if the energy is going down each cycle, or just oscillating. It looks as though

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your job below is almost converged, but it is hard to say without seeing the cycle-to-cycle output. I would suggest doing a single point calculation first, with the default (loose) convergence criteria, then look at the orbital populations (I would suggest pop=nbo for this). It is possible that you need to rearrange the orbitals if Gaussian has used a bad guess. Use guess=(read,alter) for this. The easiest thing you can try is to use scf=qc (for quadratic convergence) which, though it can take longer, is more likely to finally converge. You can also crank up the maxcycles some more. Also, once you have the first, converged SCF wavefunction, things usually settle down. This is another good reason to do a single point first. Backup the checkpoint file, and now you can try moving orbital populations around without having to duplicate the long, slow first SCF. Just starting an optimization without looking at a single point first is not a good idea. Especially if you are sharing the cpu's.

Hope that helps - John

------------------------------------------------------------ gaussian.com!csd@gaussian.com (Cust. Service Doug) Dr. Shan,

SCF convergence for systems with metals is much more challenging than for organics. This is made worse for unsaturated systems where there are often low lying virtual orbitals which would support a number of different SCF solutions within a narrow energy range. There are on going development efforts to improve this but in the mean time there are some comments I can make. First, whenever you encounter SCF convergence failures you need to look at the progress of the SCF by using #P in place of # which will print out the status at each iteration. It is useful to know if the SCF is converging slowly, oscillating, taking large jumps etc. Slow convergence can be a sign that a higher order converger like SCF=QC would be helpful. Oscillations can be a sign that a small HOMO/LUMO gap is causing a flip in the occupied orbitals and SCF=VShift=n is useful. Large jumps can indicate that a poor initial guess or a change in the electronic state from the initial guess has occured and normal convergence will eventually achieved but after a larger number of cycles and a better initial guess would help. Second, whenever possible it is helpful to start with a smaller basis set, i.e. STO-3G, LANL1MB, etc for single points. This is cheaper and so you can often generate a better initial guess which can be used with GUESS=READ and the larger basis set and without the more expensive options. In your case the SCF converges initially quite smoothly and then bounces around without making progress. This makes SCF=QC a likely approach to clean up this problem, at least at the initial point. You might want to look at adding IOP(5/13=1) which allows the SCF to print out the MO's even in the event that convergence was not reached. This should only be used with single points, i.e. remove OPT. As to optimization constraints. To freeze atomic coordinates you should turn on NoSymm. This method of freezing structures causes problems for redundant internals which are being looked into but NoSymm fixes the most serious ones.

How to start and run a simple calculation with Gaussian

You need the coordinates, charge, and multiplicity. The first line specifies your job, i.e. the method, the task (), and the basis set. Methods: HF, MP2, MP3, B3LYP, AM1, PM3, CCSD(T), etc.

Basis sets: most basis sets can be requested by their normal names, e.g. Sto-3G, 6-311+G(2d,2p).

Tasks: Opt = geometry optimization; Freq = frequency calculation; no option = single-point energy calculation. Gaussian is not case sensitive, You should start the line by #P indicating that you will have more than minimum output. Example:

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