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Solve common Gaussian software errors with clear explanations and practical fixes. This guide covers frequent error messages in Gaussian quantum chemistry calculations, explains their causes, and provides step-by-step solutions to help researchers, students, and computational chemists troubleshoot efficiently and keep simulations running smoothly.
Below are some common errors and their solutions.
This is an input error. Keyword or syntax error(s) occurred in input file, the error is marked with ‘.
This is an input error. Gaussian can not find the Z-matrix. There are two common causes:
* You may have omitted the blank line at the end of the geometry specification.
* You may have intended to get the Z-matrix and parameters from the checkpoint file, but forgot to type geom=check.
This is an input error. Gaussian can not interpret charge/multiplicity line: ask for an integer but a string was given. There are two common causes:
You may forget to write the charge/multiplicity line.
If the title line is forgotten when using geom=modify, then Gaussian interprets the charge/multiplicity line as the title, and then tries to interpret the variable list as the charge/multiplicity line.
Fix: Check your input file, add the charge/multiplicity line or title.
Performing calculations that require restricted optimization (e.g., F and S with opt=modredundant, QST2, etc.), the Optimizer does not know how to take the structural initial guesses under the current constraints.
If QST2 is used, try TS(Berny) or QST3.
If doing opt=modredundant calculations, use a smaller step size or modify the initial geometry.
If doing the fopt calculation, make a bit of a change to the geometry and resubmit to the calculation.
The internal coordinates have inherent limitations, and this problem may occur when several atoms line up exactly during the optimization process.
Using opt=cartesian: This method solves the problem completely in principle, but opt=cartesian increases the number of steps needed to optimize to the corresponding minima in most cases. If the system is not very time-consuming, this keyword can be used directly until the optimization converges. If the system is time-consuming, you can use opt=cartesian, after 2 or 3 optimization steps, save the optimized structure, and redo the optimization with the default opt method. Please note that this method is not suitable when using the opt=modredundant keyword.
Sometimes, re-optimizing the final structure directly can solve this problem. Gaussian actually adds some linear bend automatically for atoms close to the line, but it doesn’t always work.
This error appears to be a bug in Gaussian when performing a frequency calculation. Frequencies gave NaN instead of numbers.
Take the structure from the last step of geometry optimization and submit it for opt freq again.
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