Atomization Energies the W2-1 Set

Results using W1 and W2 theories are shown in Table 2.1. For W2 theory we find a mean absolute deviation (MAD) of 0.23 kcal/mol, which further drops to 0.18 kcal/mol when the NO, O2, and F2 molecules are deleted (all of which have mild nondynamical correlation in common). Our largest deviation is 0.70 kcal/mol. We can hence state that W2 meets our design goals. 

Values of Ezpv derived from B3LYP/VTZ+1 harmonic vibrational frequencies scaled by 0.985. Same remark applies to W2h, Wl, Wlh and Wlc data given. d For systems where W2h and Wlh are equivalent to W2 and Wl, respectively, entries have been left blank. 

For W1 theory, MAD is increased to 0.37 kcal/mol (old SCF extrapolation) or 0.40 kcal/mol (new SCF extrapolation), with the maximum error being 0.78 kcal/mol. This should be compared with MAD of 1.25 kcal/mol for G2 theory, 0.89 kcal/mol for G3 theory, 0.88 kcal/mol for CBS-Q, and 0.61 kcal/mol for CBS-QB3, and the much higher maximum errors of these methods of 4.90 kcal/mol (SO2), 3.80 kcal/mol (SO2), 3.10 kcal/mol (OCS), and 1.90 kcal/mol (C2H2), respectively. While we would prefer to use W2 theory for no-nonsense benchmarking if at all possible, W1 theory still seems to offer great advantages over the other techniques. 

Some representative results can be found in Table 2.2. For the G2-1 set of electron affinities, W1 theory has a mean absolute error of 0.016 eV [26]. Not unexpectedly - given the slow basis set convergence of electron affinities - the extra effort invested in W2 theory pays off with a further reduction of the mean absolute error to 0.012 eV. Accuracy appears to be limited principally by imperfections in the CCSD(T) method for the atoms B-F and Al-Cl, using even larger basis sets we achieve 0.009 eV at the CCSD(T) level, which decreases to 0.001 eV if approximate full Cl energies are used. 

Some representative results can again be found in Table 2.2. At the W1 level, the G2-1 ionization potentials are reproduced with a MAD of only 0.013 eV [26], No further improvement is seen at the W2 level for this property. Note that if the B3LYP/VTZ geometry for CH4 is employed, a serious error is seen for IP(CH4) which disappears when a CCSD(T)/VTZ reference geometry is used instead. (Only BH HLYP 

---