Our group has discovered a novel low-density ice structure (called Ice-i) which is the most stable ambient-pressure ice structure for the commonly computational models for water (including the TIP3P and SPC/E water models that are used in CHARMM and Amber). We believe ice-i is an interesting and important computational artifact, but we await experimental confirmation that this is really an imaginary computer-only structure.

Image (and research) courtesy Chris Fennell