# 7. The ATSELECT language¶

Several functions (or methods) in Yaff have a selection atoms as one of the function arguments. This selection must be provided in the form of a list or array of selected atom indexes. The ATSELECT language allows one to define rules that generate such lists of atom indexes that match a certain specification.

ATSELECT is similar to SMARTS. The SMARTS system has the advantage of being very compact, but it has a few disadvantages that make it poorly applicable in the context of Yaff: e.g. it assumes that the hybridization state of first-row atoms and bond orders are known. The only real knowns for ATSELECT are: numbers and optionally ffatypes, scopes and bonds.

The syntax of the ATSELECT language is defined as follows. An ATSELECT expression consists of a single line and is case-sensitive. White-space is completely ignored. An ATSELECT expression can be any of the following:

[scope:]number
Matches an atom with the given number, optionally part of the given scope.
[scope:]ffatype
Matches an atom with the given atop type, optionally part of the given scope.
scope:*
Matches any atom in the given scope.
expr1 & expr2 [& ...]
Matches an atom that satisfies all the given expressions.
expr1 | expr2 [| ...]
Matches an atom that satisfies any of the given expressions.
!expr
Matches an atom that does not satisfy the given expression.
=N[%expr]
Matches an atom that has exactly N neighbors, that optionally match the given expression.
>N[%expr]
Matches an atom that has more than N neighbors, that optionally match the given expression.
<N[%expr]
Matches an atom that has less than N neighbors, that optionally match the given expression.
@N
Matches an atom that is part of a strong ring with N atoms. Not implemented yet.
(expr)
Round brackets are part of the syntax, used to override operator precedence. The precedence of the operators corresponds to the order of this list.

In the list above, expr can be any valid ATSELECT expression. Atom types and scope names should not contain the following symbols: :, %, =, <, >, @, (, ), &, |, !, and should not start with a digit. Some examples of atom selectors:

• 6 – any carbon atom.
• TPA:6 – a carbon atom in the TPA scope.
• C3 – any atom with type C3.
• TPA:C3 – an atom with type C3 in the TPA scope.
• !1 – anything that is not a hydrogen.
• C2|C3 – an atom of type C2 or C3.
• 6|7&=1%1 or (6|7)&=1%1 – a carbon or nitrogen bonded to exactly one hydrogen.
• >0%(6|=4) – an atom bonded to at least one carbon atom or bonded to at least one atom with four bonds.
• 6&@6 – a Carbon atom that is part of a six-membered ring.

There are currently three ways to use the ATSELECT strings in Yaff:

1. Compile the string into a function and use it directly:

from yaff import *
fn = atsel_compile('C&=4')
system = System.from_file('test.chk')
if fn(systen, 0):
pass
# Do something if the first atom is a carbon with four neighbors.
# ...

2. Get all atom indexes in a system that match a certain ATSELECT string:

from yaff import *
system = System.from_file('test.chk')
indexes = system.get_indexes('C&=4')
# The array indexes contains all the indexes of the carbon atoms with
# four neighbors.

3. Define FF atom types in a system based on ATSELECT strings. For this purpose, one can normally not rely on the presence of FF atom types in the system object.

from yaff import *
system = System.from_file('test.chk')
system.detect_ffatypes([
('H', '1'),
('O_water', '8&=2%1'),
])


Whenever one uses a compiled expression on a system that does not have sufficient attributes, a ValueError is raised. For example, a ValueError would be raised when one would refer to atom types in the third use case.