W2X and W3X-L: Cost-Effective Approximations to W2 and W4 with kJ mol(-1) Accuracy.

We have formulated the W2X and W3X-L protocols as cost-effective alternatives to W2 and W3/W4, respectively, and to supplement our previously developed set of W1X and W3X procedures. The W2X procedure provides an accurate approximation to the all-electron scalar-relativistic CCSD(T)/CBS energy, with a mean absolute deviation (MAD) of 0.6 kJ mol(-1) from benchmark energies provided by the CCSD(T) component in the W4 protocol. Such a performance is comparable to that of W2w (0.5 kJ mol(-1)) but comes at a significantly lower cost. Comparison of computational requirements shows that W2X should be applicable to systems that can be treated by the W1w method. Thus, W2X provides an accurate means for the treatment of medium-sized systems such as naphthalene. For the calculation of post-CCSD(T) effects, we propose a slight modification to the method used in our previously devised W3X procedure. Our new W3-type protocol (W3X-L) combines this new post-CCSD(T) treatment with our new W2X procedure. It has an MAD from benchmark values of 0.8 kJ mol(-1) for the W4-11 set, which is comparable to that for the computationally more demanding W3.2 method (0.6 kJ mol(-1)). However, the use of the even relatively modest post-CCSD(T) calculations in W3X-L still represents a computational bottleneck, and this currently restricts its application to systems up to the size of benzene with our current computing resources.