CurlySMILES annotation dictionary key: spg

The lower-case three-character notation spg is used as an annotation dictionary key specifying a space group symbol. For example, crystalline phosphorus in the space group Cmca (black phosphorus) can be encoded in CurlySMILES as

[P]{crspg=Cmca}

and zinc sulfide in the space groups F43m (zincblende) and P6₃mc (wurtzite) as

[Zn+2].[S-2]{crspg=F-43m} or {*ZnS}{crspg=F-43m}
[Zn+2].[S-2]{crspg=P6_3mc} or {*ZnS}{crspg=P6_3mc}

CurlySMILES applies a linearized format of the standard, internationally recognized Hermann-Mauguin symbols to designate the 230 space groups. Presentations of the Hermann-Mauguin symbols are available with the Bilbao Crystallographic Table of Space Group Symbols and at the IUCr site (DOI: 10.1107/97809553602060000100).

The encoding examples above illustrate that the linearized format in CurlySMILES uses a dash in front of a digit to denote the overbar-digit and an underscore in front of a digit to indicate its subscript appearance. White space is not allowed. The complete set of linearized notations, applicable to assign a value to key spg in a CurlySMILES annotation, is given in the following table, along with their marked-up versions, in the order of their space group numbers.