Extraction of molecular edges on the average difference map by a Monte-Carlo method

A method giving a low-resolution image of the molecules in the unit cell has been described, which was based only on the observed structure factors. An operator, called the average difference operator (ADO), was introduced in reciprocal space to flatten the electron densities everywhere but the regions on either side of the molecular envelope in real space. The observed structure factors were first modified by ADO, then a Monte-Carlo condensing protocol [Subbiah (1991). Science, 252, 128-133; (1993). Acta Cryst. D49, 108-119] was employed to stimulate the modified electron-density map at low resolution. It was found that molecular edges could be extracted, especially when there was relatively large solvent content in the unit cell.