Deprecated: please see new documentation site.


There are a few common Gaussian problems that can be easily resolved. These issues usually stem from disk or memory space limitations. First one needs to understand the basic run-time needs of Gaussian calculations. The table below is the Formal Scaling Behavior of Gaussian, in which N = the number of basis functions.

Scaling Behavior Method(s)
N4 HF
N5 MP2
N6 MP3, CISD, CCSD, QCISD
N7 MP4, CCSD(T), QCISD(T)
N8 MP5, CISDT, CCSDT
N9 MP6
N10 MP7, CISDTQ, CCSDTQ

Large files and memory usage

Computational cost and demand increases quickly when trying to obtain accuracies better than the MP2 level. On the other hand, one can supply a large molecule at a lower level of theory and still come across the same disk/memory errors.

If one has a large model and needs a good electron correlation method, starting this calculation from an initial guess wave function will likely cause it to fail instantly. A typical route in achieving such accuracies with a large model begins with a good initial guess of the wave function at a lower level of theory. In this method, one uses the orbital coefficients from the lower level of theory calculations, projects them onto a larger basis set, and uses that as an initial guess for the high level of theory. Every chemical model is different; care and caution needs to be taken at each step, perhaps even repeat the calculation using a different set of inputs to see if it converges properly.

For instance, if one would like to run a large model at the MP2/6-311G** level of theory.

1) Optimize wave function at the HF/3-21G

2) Re-optimize at the MP2/6-31G*

3) Re-optimize at MP2/6-311G**

When restarting the calculation the following Guess options are important

Guess=Read

Reads the initial guess from the checkpoint file. If the basis set specified is different from the basis set used in the job which generated the checkpoint file, then the wave function will be projected from one basis to the other. This is an efficient way to switch from one basis to another.

Geom=AllCheckpoint

Reads the molecular geometry, charge, multiplicity and title from the checkpoint file. This is often used to start a second calculation at a different level of theory.

Also needed, SCF=Restart (to use the checkpoint file).

Break up restart files

Sometimes when writing a large restart file, Gaussian will crash complaining about shared memory is too small, or not enough memory. This is caused by reading/writing too much information at one time. One can break up how it writes its read-write restart file (*.rwf) by %rwf =/work/username/tmp1,2GB,/work/username/tmp2,2GB,/work/username/tmp3,2GB (if the last file doesn't have a number, then the rest of the rwf is written to that file).

One problem, two solutions

If one experiences two different solutions to the same problem- either same calculation on two different machines or same calculation ran at different times on the same machine. One is likely using the incorrect restart file. Check your output calculations- namely the NOrb value (different amount of orbitals, likely a different energy result).

For more information on memory and disk space usage, please read http://www.gaussian.com/g_tech/g_ur/m_eff.htm

Warnings should not be ignored

Warning!!: The largest alpha MO coefficient is

This warning is usually associated with post-HF calculation (MP2 or CC). Although, this is not an error will and will not cause your job to crash, it is an important warning. It warns on the accuracy of your calculation. This occurs when one has a near-linear dependencies in the basis sets. For instance, diffuse functions on two close atoms are likely linearly dependent. When transforming to molecular orbitals, the atomic orbital integrals are multiplied by all the molecular orbital coefficients. The accuracy of the molecular orbital will decrease since one or more atomic orbitals are very large.

Powered by MediaWiki