Dear All<br><br>I like to make a graphene bilayer. I wanted to use  &quot;honeycomb&quot; definition in the lattice library but I found something<br>strange! The strange thing is the definition of vertexes:<br><br>  &lt;UNITCELL name=&quot;honeycomb&quot; dimension=&quot;2&quot;&gt;<br>
  &lt;VERTEX&gt;&lt;COORDINATE&gt;0 0&lt;/COORDINATE&gt;&lt;/VERTEX&gt;<br>  &lt;VERTEX&gt;&lt;COORDINATE&gt;-0.288665771484375 0.5773468017578125&lt;/COORDINATE&gt;&lt;/VERTEX&gt;<br>  .......<br>   ......<br>  <br>   &lt;/UNITCELL&gt;<br>
<br>I don&#39;t know how these two numbers &quot;-0.288665771484375 0.5773468017578125&quot; were generated ( they are near to &quot;-1/(2*sqrt(3)), 1/sqrt(3)&quot; !)<br>I think if we suppose the vertex coordinate is defined according to the lattice vectors (.i.e. a1=[1  0], a2=[1/2 sqrt(3)/2]),<br>
then the correct numbers should be: &quot;-1/3, 2/3&quot; (-1/3*a1+2/3*a2). So the correct &quot;UNITCELL&quot; should be:<br><br><br>  &lt;UNITCELL name=&quot;honeycomb&quot; dimension=&quot;2&quot;&gt;<br>
  &lt;VERTEX&gt;&lt;COORDINATE&gt;0 0&lt;/COORDINATE&gt;&lt;/VERTEX&gt;<br>
  &lt;VERTEX&gt;&lt;COORDINATE&gt;-1/3  2/3&lt;/COORDINATE&gt;&lt;/VERTEX&gt;<br>
  .......<br>
   ......<br>
  <br>
   &lt;/UNITCELL&gt;<br><br>I checked the new &quot;UNITCELL&quot;, and I found the lattice is more like to graphene.<br>If I am wrong please correct me. <br><br>Thanks<br><br>Kaveh Edalat<br><br>