We have quantified the effects of approximations usually made even in accurate CCSD(T)/CBS calculations of noncovalent interactions, often considered as the "gold standard" of computational chemistry. We have investigated the effect of excitation series truncation, frozen core approximation, and relativistic effects in a set of 24 model complexes. The final CCSD(T) results at the complete basis set limit with corrections to these approximations are the most accurate estimate of the true interaction energies in noncovalent complexes available. The average error due to these approximations was found to be about 1.5% of the interaction energy.

Institute of Organic Chemistry and Biochemistry Academy of Sciences of the Czech Republic 166 10 Prague Czech Republic

Regional centre of Advanced Technologies and Materials Department of Physical Chemistry Palacky University 771 46 Olomouc Czech Republic

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