DFIX Instruction

DFIX d s [#] atom pairs

The distance between the first and second named atom, the third and fourth, fifth and sixth etc. (if present) is restrained to a target value d with an estimated standard deviation s. d may refer to a 'free variable', otherwise it is considered to be fixed. Fixing d by adding 10 is not allowed, so the value may lie between 0 and 15. If d is given a negative sign, the restraint is applied ONLY if the current distance between the two atoms is LESS than |d|. This is an 'antibumping' restraint, and may be used to prevent solvent (water) molecules from approaching too close to one another or to a macromolecule. Antibumping restraints may also be generated automatically using the BUMP instruction (see below). The default value of s is 0.03 when d is positive and 0.1 when d is negative. The default s for positive d may be changed by means of a preceding DEFS instruction (see below).