# Selection language¶

Chemfiles selection language allows to select some atoms in a Frame matching a set of constraints. For examples, atom: name H and x > 15 would select all single atoms whose name is H and x coordinate is bigger than 15.

Chemfiles selections differs from the well-known VMD selections by the fact that they are multiple selections: we can select more than one atom at once. All selections starts with a context, indicating the number of atoms we are selecting, and the relation between these atoms. Existing contextes are atoms or one, pairs or two, three and four to select one, two, three or four independent atoms; and bonds, angles and dihedrals for two, three or four bonded atoms.

A selection is built using a context and a set of constraints separated by a colon. For example, atoms: name == H will select all atoms whose name is H. angles: name(#2) == O and mass(#3) < 1.5 will select all sets of three bonded atoms forming an angle such that the name of the second atom is O and the mass of the third atom is less than 1.5.

These constraints are created using selectors. Selectors are small functions that are evaluated for each atom, and return either true if the atom matches, or false if it does not. There are three kinds of selectors:

• boolean selectors returns either true or false for a given set of atoms;
• string selectors compare string values with one of == (equal) or != (not equal). One can either compare two atomic properties (name(#1) == type(#2)) or atomic properties to literal strings (name(#1) != He);
• numeric selectors compare two numeric values with either ==, !=, < (less than), <= (less or equal), > (more than), and >= (more or equal)

Numeric values are produced by numeric selectors (x; mass, …) or literal values (5.2, 22.21e-2). They can also be combined together using mathematical operations: the usual +, -, * and / operators are supported, as well as ^ for exponentiation. These operations follow the usual priority rules: 1 + 2 * 3 is 7, not 9.

When using a selection with more than one atom, selectors must refer to the different atoms with #1, #2, #3 or #4 variables: name(#3) will give the name of the third atom, and so on.

Finally, constraints are combined with boolean operators. The and operator is true if both side of the expression are true; the or operator is true if either side of the expression is true; and the not operator reverse true to false and false to true. name(#1) == H and not x(#1) < 5.0 and (z(#2) < 45 and name(#4) == O) or name(#1) == C are complex selections using booleans operators.

## List of implemented selectors¶

Here is the list of currently implemented selectors. Additional ideas are welcome!

### Boolean selectors¶

• all: always matches any atom and returns true;
• none: never matches an atom and returns false;
• is_bonded(i, j): check if atoms i and j are bonded together. If i and j refers to the same atom, this returns false;
• is_angle(i, j, k): check if atoms i, j and k are bonded together to form an angle, i.e. that i is bonded to j and j is bonded to k. If any of i, j or k refer to the same atom, this returns false;
• is_dihedral(i, j, k, m): check if atoms i, j, k and m are bonded together to form a dihedral angle, i.e. that i is bonded to j, j is bonded to k, and k is bonded to m. If any of i, j, k or m refer to the same atom, this returns false;
• is_improper(i, j, k, m): check if atoms i, j, k and m are bonded together to form a dihedral angle, i.e. that all of i, k, and m are bonded to j. If any of i, j, k or m refer to the same atom, this returns false;

For boolean selectors taking arguments, i/j/k/m can either be one of the atoms currently being matched (#1 / #2 / #3 / #4) or another selection (called sub-selection). In the latter case, all the atoms in the sub-selection are checked to see if any of them verify the selection. This makes is_bonded(#1, name O) select all atoms bonded to an oxygen; and is_angle(type C, #1, name O) select all atoms in the midle of a C-X-O angle.

### String properties¶

• type: gives the atomic type;
• name: gives the atomic name. Some formats store both an atomic name (H3) and an atom type (H), this is why you can use two different selectors depending on the actual data;
• resname: gives the residue name. If an atom is not in a residue, this return the empty string;

### Numeric properties¶

Most of the numeric properties only apply to a single atom:

• index: gives the atomic index in the frame;
• mass: gives the atomic mass;
• x, y and z: gives the atomic position in cartesian coordinates;
• vx, vy and vz: gives the atomic velocity in cartesian coordinates;
• resid: gives the atomic residue index. If an atom is not in a residue, this return -1;

But some properties apply to multiple atoms, and as such are only usable when selecting multiple atoms:

• distance(i, j): gives the distance in Ångströms between atoms i and j, accounting for periodic boundary conditions.
• angle(i, j, k): gives the angle between atoms i, j and k in radians, accounting for periodic boundary conditions. The atoms do not need to be bonded together.
• dihedral(i, j, k, m): gives the dihedral angle between atoms i, j, k and m in radians, accounting for periodic boundary conditions. The atoms do not need to be bonded together.
• out_of_plane(i, j, k, m): gives the distance in Ångströms between the plane formed by the three atoms i, k, and m; and the atom j, accounting for periodic boundary conditions.

Note

The angle and dihedral selectors are different from the is_angle and is_dihedral selectors. The firsts returns a number that can then be used in mathematical expressions, while the second returns directly true or false.

One can also use mathematical function to transform a number to another value. Currently supported functions are: deg2rad and rad2deg functions for transforming radians to degrees and respectively; sin, cos, tan for the trigonometric functions; asin and acos inverse trigonometric functions and sqrt. Adding new functions is easy, open an issue about the one you need on the chemfiles repository.

## Elisions¶

This multiple selection language can be a bit verbose for simpler cases, so it is sometimes allowed to remove parts of the selection. The following rules allow simpler selections:

• First, in the atoms context, the #1 variable is optional, and atoms: name(#1) == H is equivalent to atoms: name == H.
• Then, if no context is given, the atoms context is used. This make atoms: name == H equivalent to name == H.
• Then if no comparison operator is given, == is used by default. This means that we can write name H instead of name == H.
• Then, multiple values are interpreted as multiple choices. A selection like name H O C is expanded into name H or name O or name C.

At the end, using all these elisions rules, atom: name(#1) == H or name(#1) == O is equivalent to name H O. A more complex example can be bonds: name(#1) O C and index(#2) 23 55 69, which is equivalent to bonds: (name(#1) == O or name(#1) == C) and (index(#2) == 23 or index(#2) == 55 or index(#2) == 69)