EDITORIAL

In this issue of Molecular Physics, we are pleased to present the latest article in our on-going series of Invited Topical Reviews. This paper is co-authored by Toru Shiozaki and Joachim Werner, on ‘Multireference explicitly correlated F12 theories’ [1], covering an exciting recent development in high-accuracy electronic structure calculations. F12 methods [2,3] are a rapidly developing approach to explicitly treating electron correlations. They work by including 2-particle functions of the inter-electronic distance, R12, in the basis, in contrast to the usual 1-particle atomic orbital basis sets, and are a robust alternative to widely used extrapolation methods, whose challenges have recently been discussed [4].

F12 methods [2,3] have usually been applied to single-reference electronic structure methods. For instance, Werner and collaborators have published several papers in this direction in Molecular Physics recently [5,6]. However, it is well known that multi-reference approaches, such as multi-reference configuration interaction (MRCI) and complete active space second order perturbation theory (CASPT2) [7], are essential for problems where different electron configurations become close in energy. They also permit the treatment of excited states in such systems. Recent papers in our journal employing CASPT2 and MRCI include refs. [8–11]. The F12 advances described in ref. [1] have direct potential for improving accuracy relative to efficiency for advanced applications of CASPT2 and MRCI.

Within the Topical Reviews series, other recent publications with a focus on electronic structure include a review on the random phase approximation as a treatment of electron correlation, by Heßelmann and Gorling (Erlangen) [12], a comprehensive discussion of relativistic effects in electronic structure, by Liu (Peking) [13], and an overview of high-accuracy quantum chemistry, by Helgaker et al. [14]. Other recent Topical Reviews articles cover the quantum theory of reaction rates [15], polarisation effects in ionic solids and melts [16], photoelectron angular distributions in molecules [17], the hierarchical reference theory of fluids [18], the structure and dynamics of water [19,20], as well as many others.

Readers with a focus on electronic structure theory may be interested in recent Special Issues of Molecular Physics specifically devoted to in this area. Particularly relevant is the Special Issue (issues 3–4 of vol. 108) in honour of the co-author of ref. [1], and current Molecular Physics Editor, Joachim Werner [21], which contains interesting background on his scientific career, as well as an excellent collection of original research papers. In 2012, one Special Issue (numbers 19, 20 of vol. 110) was dedicated in honour of Prof. Peter Taylor [22]. Other relevant Special Issues include the 50th Anniversary of the Quantum Theory Project (QTP) (vol. 108, numbers 21–23), and the proceedings of Molecular Quantum Mechanics 2010 (vol. 108, numbers 19–20) in honour of former Molecular Physics Editor and current Board Member, Henry ‘Fritz’ Schaefer.

Molecular Physics continues to encourage the submission of original research papers in all areas of electronic structure theory, as well as suggestions for future appropriate special issues in the field.