1. Could you explain the significance of your article to the non-specialist?

The CCSD(T) electronic structure method is a standard tool for computational chemists, providing reliable predictions of molecular energies and properties, often with errors similar to or smaller than those of experiment. However, large numbers of orbital basis functions are required to realise the inherent accuracy of the method, making these calculations very expensive. The new CCSD(T)(F12) method, presented in this article, yields accurate CCSD(T) energies using relatively small orbital basis sets. It can therefore be applied to probe much larger systems than is currently feasible with the conventional CCSD(T) method.

2. What has motivated you to conduct this work?  

The slow convergence of wave function methods with the size of the orbital basis has long troubled quantum chemists. However, recent progress in the explicitly-correlated R12 methods, introduced by Kutzelnigg and co-workers, indicates that, for MP2, this problem has effectively been solved, culminating in the MP2-F12 method. We were excited about the possibility that the success of MP2-F12 could be combined with the existing R12 coupled-cluster methods. 

3. Where do you see this work developing in the future? 

It is becoming clear that F12 wave function methods will eventually replace the existing orbital-only analogues as the standard tools for accurate model chemistry. Work has already begun, in several research groups, to develop F12 orbital basis sets, to implement analytic gradients and molecular response properties and to improve efficiency through RI approximations and local-correlation methods.

4. Are there any particular challenges facing future research in this area? 

The computational community will only adopt explicitly-correlated methods when they have obtained black-box status. Although much progress has already been made in understanding and refining the approximations implicit to R12 theory, more work is required before we can simultaneously claim efficiency and complete control of the associated errors.