单字符串和多字符串用来命名variable的name时，引用时注意，如果是单字符串，只需$x,而多字符串，则需加上curly brackets(花括号)，${temp}。

lammps-manual-3.2中原话如下：

 If the $ is followed by curly brackets, then the variable name is the text inside the curly brackets. If no curly brackets follow the $, then the variable name is the single character immediately following the $. Thus ${myTemp} and $x refer to variable names "myTemp" and "x"