ON THE EFFECT OF CORE CORRELATION ON THE GEOMETRY AND HARMONIC FREQUENCIES OF SMALL POLYATOMIC-MOLECULES

The effect of core correlation on computed properties of a set of experimentally well studied first-row compounds has been investigated using augmented coupled cluster (CCSD(T)) methods and specially tailored one-particle basis sets. Core correlation accounts for virtually all the remaining error in bond lengths, but simple additivity corrections based on the bond order absorb essentially all such effects starting from [4s3p2d1f] basis sets. There are nontrivial effects on harmonic frequencies, but these do not lead to improved agreement with experiment due to an error compensation between neglect of core correlation and residual n-particle space inadequacies. Finally, while significant (up to 2.5 kcal/mol) effects on total atomization energies are seen, these are essentially completely absorbed in the three-term basis set incompleteness correction proposed by the author.