Convergence FAQ

The algorithms in Spartan will typcially perform well on most systems when solving quantum mechanical equations. However, there are times when one may see "convergence problems". What to do when this occurs? The following covers the typical areas to examine when a calculation has trouble converging.

The very first thing to try is to select the the "Converge" check box in the calculations dialogue. This uses slower but more stable algorithms with broader tolerances; this may help some difficult systems to converge. If this fails, it is necessary to carefully examine what is going on. Read this document in its entirety. The main sections are listed below:



General Issues

Some common convergence issues: If you've made it this far, you have examined your molecule and believe everything is correct in the calculation setup. Now it's time to delve deeper into the calculation. The first thing to do is determine whether problems are related to the geometry optimization or the self consistent field calculation (SCF) of the wavefunction. If you have one or more geometry cycles displayed in the output dialogue, the problem is likely related to geometry optimization. (Of course, there may be exceptions, so examine the error message carefully.)

The following section includes several references to "keywords". Keywords are type-in options, to be entered in the "Options" field in Spartan's calculation dialogue. Keywords are NOT case sensitive. Multiple keywords are separated by spaces.

Geometry Optimization Issues

Wavefunction (SCF) Convergence Issues

Gaining insight into SCF convergence issues is more difficult. The non-linear Schrodinger equation and the non-locality of electrons make it difficult to understand what is actually occurring. It is often useful to request progress data from the convergence process. This can be done by using the PRINTLEV=2 keyword, or examining the "Verbose Output" if you are using Q-Chem algorithms (large molecule HF calculations and DFT and higher calculations use computational code from Q-Chem, Inc.). Examining this output can yield several clues- you should see both the energy and the "DIIS error" slowly decrease. We suggest you try some of the steps below and observe their results in the verbose output.

Concluding Remarks

Computational Chemistry is a developing discipline. Quantum chemical calculation methods have matured to a level such that programs like Spartan can routinely provide results for molecular geometries, energies, and a host of calculated properties at a predictable and useful level of accuracy with very little user intervention. Chances are high that if you spend some time with the hints mentioned above, you will overcome any computational obstacles that your specific system presents. Since it is likely that you are focused on a finite class of molecules, when you uncover the approach that works on one system, it is likely that this will also work on similar molecules.

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Other Questions

  1. I ran a geometry optimization followed by a frequency calculation and it shows a negative eigenvalue. Doesn't that imply that I've found a transition state?

    Yes it does, If the gradient is zero. There are a few likely causes of this behavior:

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  2. A geometry optimization ran out of cycles. How do I restart?

    Simply resubmitting the job will continue the optimization. If you believe it will continue to take a lot of cycles, you can increase the maximum number of cycles with the GEOMETRYCYCLES= keyword. Running out of cycles usually implies that you had a bad starting guess, or that some unexpected chemistry is occurring (such as bond breaking). Review the Geometry optimization section for a more thorough discussion on what can go wrong and how to fix it.

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Keywords mentioned

SCF=UNRESTRICTED, SCF=RESTRICTED, UHF
HESS=UNIT
GUESS=CORE
SCFTOLERANCE=x
PRINTLEV=2
GEOMETRYCYCLES=999, OPTCYCLE=999
HESS=UNIT
SCFTOLERANCE=x
SCFCYCLE=xxx
THRESH=x *
VARTHRESH=x *
GUESS=CORE
DIIS=x, NODIIS
BIGMOLECULE, SMALLMOLECULE
USEPSEUDO, NOPSEUDO *
DAMP=x, NODAMP *
SCF_ALGORITHM=x *
BIGGRID *
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Wavefunction Support
Author: Phil Klunzinger
Last modified: Wed May 9 15:47:50 PST 2003