Ab initio thermochemistry beyond chemical accuracy for first-and   second-row compounds
  By judicious use of extrapolations to the 1-particle basis set limit and $n$-particle calibration techniques, total atomization energies of molecules with up to four heavy atoms can be obtained with calibration accuracy (1 kJ/mol or better, on average) without any empirical correction. For the SCF energy a 3-point geometric extrapolation is the method of choice. For the MP2 correlation energy, a 2-point $A+B/(l+1/2)^3$ extrapolation is recommended, while for CCSD and CCSD(T) correlation energies we prefer the 3-point $A+B/(l+1/2)^C$ formula. Addition of high-exponent `inner polarization functions' to second-row atoms is essential for reliable results. For the highest accuracy, accounts are required of inner-shell correlation, atomic spin-orbit splitting, anharmonicity in the zero-point energy, and scalar relativistic effects. 
Jan M.L. Martin