Accurate geometries from ab initio calculations: systems containing second-row atoms and transition metals

The performance of the standard hierarchies of ab initio models HF, MP2, CCSD and CCSD(T) and of correlation consistent basis sets is analyzed in determining bond lengths and bond angles on a set of 32 closed-shell molecules containing second-row elements. The importance of core correlation is assessed using hierarchies of correlation consistent valence and core-valence basis sets, the former used in connection with the frozen-core approximation and the latter freezing only the 1s orbital on the second row atom. Results of the geometry optimization of ferrocene in the eclipsed and staggered conformational forms at the MP2, CCSD and CCSD(T) levels in a TZV2P+f basis set and with 66 valence electrons correlated are also presented.