Local correlation methods with a natural localized molecular orbital basis

Natural localized molecular orbitals (NLMOs) have been used in local correlation calculations. For a test set of 30 molecules it is shown that the results obtained with NLMOs and Pipek–Mezey localization are very similar. However, NLMOs are much less sensitive to the basis set, in particular when diffuse functions are used. Based on natural population analysis (NPA) a new method to determine the domains in local correlation methods is proposed. It is demonstrated that this yields domains that are very insensitive to the choice of the basis set. For all 30 molecules the same domains are obtained with six different basis sets ranging from cc-pVDZ to aug-cc-pVQZ. This allows one to define the local correlation methods more uniquely than with previous methods.