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• New database for assessing and developing functionals and semi-empirical methods. • Benchmarking of several popular methods for geometry optimization. • Comparison between the long bonds in CLB18 with bond lengths in transition structures. • Inclusion of effects beyond dispersion not commonly found in other structural database in the literature. We introduce a new database called Carbon Long Bond 18 (CLB18), composed of 18 structures with one long C–C bond. We use this new database to evaluate the performance of several low-cost methods commonly used for geometry optimization of medium and large molecules. We found that the long bonds in CLB18 are electronically different from those found in barrier heights databases. We also report the unexpected correlation between the results of CLB18 and those of the energetics of spin states in transition-metal complexes. Given this unique property, CLB18 can be a useful tool for assessing existing electronic structure calculation methods and developing new ones. [ABSTRACT FROM AUTHOR]