Thumbnail image of graphical abstract

NBO 6.0, the next generation in natural bond orbital methods, is released by Frank Weinhold, Eric Glendening, and Clark Landis on page 1429. NBO 6.0 includes link-free connectivity for interfaces to popular electronic structure programs, new methods of analysis, and numerous algorithmic enhancements. The cover image is made with the NBOView 2.0 program and shows the sigma-type, 3-center/2-electron bonding NBO of the cyclobutenyl cation that arises from the strong overlap of the vacant valence orbital at C1 with the C2–C3 bond. Automated multicenter bond searches are one of the many new features of the NBO program.