The description of substituent groups, attached to an organic skeleton, by means of basis sets of lower quality with respect to the rest of the molecule, has been investigated and tested. Starting from a double‐zeta Gaussian basis, two successive reductions of the basis set dimensions have been envisaged: the first is simply a contraction to single zeta, the second consists in constructing ‘‘supercontracted’’ basis functions as linear combinations of AOs centered on different atoms of the same substituent group. An optimization of contraction and ‘‘supercontraction’’ coefficients at the molecular level leads to much better results for the energy and other molecular properties, in comparison with the common practice of using the contraction based on atomic calculations. An unbalanced description, potentially implied in using basis sets of different quality in the same calculation, is thus avoided. The example of the methyl group shows that the contraction coefficients determined for a neutral molecular environment (the ethane molecule) can be used in SCF‐CI calculations with different aims: for example, the study of the electronic excitation in acetone, of the protonation in amines, of radical and carbocation formation in alkanes
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