Attributes describe the properties of the elements, therefore each
attribute cannot appear more than once for each element. Attributes
can be written in any order and separator characters (space, tab, newline) 
before or after the attributes or their values are ignored.
Some attributes are required (for example, the 
x, 
y,
z atom coordinates), some are optional (for example the atom 
mass) and default values are used when they are not explicitely 
indicated. What are these default values and how to change them is the 
subject of the 
Gamgi part of this manual.
Some optional attributes are coupled (for example, the 
red, 
green, 
blue atom colors), which means that, if present, the whole set must
be indicated.
Some attributes are incompatible because they describe properties that cannot 
be valid simultaneously for the same object (for example, a cell element cannot 
have simultaneously 
n1, 
n2, 
n3 number of cells and a volume 
set to 
sphere).
For each GAMGI object, a description of all the accepted attributes,
with their possible values and restrictions, is presented in the 
corresponding chapter of this manual.
Good:
<atom element="H" x="0.0" y="0.0" z="0.0"/> all required attributes are present
<atom 
      z=  "0.0" 
      y="  0.0" 
      x="0.0  "
      element="H" /> equivalent to example above
<atom element="H" x="0.0" y="0.0" z="0.0" mass="2.0"/> optional attributes may be added
Bad:
<atom element="H" x="0.0" z="0.0"/> a required parameter is missing
<atom element="H" x="0.0" y="0.0" z="0.0"
red="1.0" blue="1.0" /> a coupled parameter is missing
<atom element="H" x="0.0" y="0.0" z="0.0" element="H"/> the same attribute appears twice
<cell system="c" lattice="P" a="1.0" n1="1" n2="1" n3="1" volume="sphere" v1="1.0"/> 
incompatible attributes