Selection Methods

VMD has a rather powerful atom selection language available. It is based around the assumption that every atom has a set of associated with it values which can be accessed through keywords. These values could be boolean (is this a protein atom?), numeric (as in the atom index or atomic mass), or string (the atom name). The values can even be referenced via a Tcl array.

To start off, here are some examples of valid selection commands in VMD. Following these will be a more in depth description of how selections work.

        name CA
        resid 35
        name CA and resname ALA
        backbone
        not protein
        protein (backbone or name H)
        name 'A 1'
        name 'A *'
        name "C.*"
        mass < 5
        numbonds = 2
        abs(charge) > 1
        x < 6 and x > 3
        sqr(x-5)+sqr(y+4)+sqr(z) > sqr(5)
        within 5 of name FE
        exwithin 3 of protein
        protein within 5 of nucleic
        same resname as (protein within 5 of nucleic)
        protein sequence "C..C"
        name eq $atomname 
        protein and @myselection

There are two types of selection modes. The first is a keyword followed by a list of either values or a range of values. For example,

        name CA

selects all atoms with the name CA (which could be a C${}_\alpha$ or a calcium);

        resname ALA PHE ASP

selects all atoms in either alanine, phenylalanine, or asparagine;

        index 5

selects the 6th atom (in the internal VMD numbering scheme).

VMD can also do range selections, similar to X-PLOR's `:' notation:

        mass 5 to 11.5

selects atoms with mass between 5 and 11.5 inclusive,

        resname ALA to CYS TYR

selects atoms in alanine, arginine, asparagine, aspartic acid, cystine, and also tyrosine.

The keyword selection works by checking each term on the list following the keyword. The term is either a single word (eg, name CA) or a range (eg resid 35 to 90).

The method for determining the range checking is determined from the keyword data type; numeric comparisons are different than string comparisons. The comparison should work as expected so that ``8'' is between ``1'' and ``11'' in a numeric context but not in a string one. This may lead to some peculiar problems. Some keywords, such as segname, can take on string values but can also be used by some people as a number field. Suppose someone labeled the segname field with the numbers 1 through 12 on the assumption that they are numbers. That person would be rather confused to find that segname 1 to 11 only returns two segments. Also, strings will be converted (via atof()) to a number so if the string isn't a number, it will be given the value of 0. It is possible to force a search to be done in either a string or numeric context using the relational operator

Selections can be combined with the boolean operators and and or, collected inside of parenthesis, and modified by not, as in

        (name CA or name CB) and mass 12 to 17

which selects all atoms name CA or CB and have masses between 12 and 17 amu (this could be used to distinguish a C-alpha from a calcium). VMD has operator precedence similar to C so leaving the parentheis out of the previous expression, as in:

        name CA or name CB and mass 12 to 17

actually selects all atoms named CA or those that are named CB and have the appropriate mass.