Atom Expressions

RasMol atom expressions uniquely identify an arbitrary group of atoms within a molecule. Atom expressions are composed of either primitive expressions, predefined sets, comparison operators, within expressions, or logical (boolean) combinations of the above expression types.

The logical operators allow complex queries to be constructed out of simpler ones using the standard boolean connectives and, or and not. These may be abbreviated by the symbols "&", "|" and "!" respectively. Parentheses (brackets) may be used to alter the precedence of the operators. For convenience, a comma may also be used for boolean disjunction.

The atom expression is evaluated for each atom, hence protein and backbone selects protein bacbone atoms, not the protein and [nucleic] acid backbone atoms!

Examples:    backbone and not helix
             within( 8.0, ser70 )
             not (hydrogen or hetero)
             not *.FE and hetero
             8, 12, 16, 20-28
             arg, his, lys