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 ce ll has been described, which was based only on the observed structure factors. An operator, called the average difference operator (ADO), wa s introduced in reciprocal space to flatten the electron densities eve rywhere but the regions on either side of the molecular envelope in re al space. The observed structure factors were first modified by ADO, t hen a Monte-Carlo condensing protocol [Subbiah (1991). Science, 252, 1 28-133; (1993). Acta Cryst. D49, 108-119] was employed to stimulate th e modified electron-density map at low resolution. It was found that m olecular edges could be extracted, especially when there was relativel y large solvent content in the unit cell.