Dear Cathy,
The Michele's setting was sufficient for you, but there is another way of making inhomogeneous lattice. You can use "INHOMOGENEOUS" keyword in the lattice setting. In mylattice.xml, you can define the graph such as:
<LATTICES> <LATTICE name="chain lattice" dimension="1"> <PARAMETER name="a" default="1"/> <BASIS><VECTOR>a</VECTOR></BASIS> <RECIPROCALBASIS><VECTOR>2*pi/a</VECTOR></RECIPROCALBASIS> </LATTICE>
<UNITCELL name="simple1d" dimension="1"> <VERTEX/> <EDGE> <SOURCE vertex="1" offset="0"/> <TARGET vertex="1" offset="1"/> </EDGE> </UNITCELL>
<LATTICEGRAPH name = "inhomogeneous open chain lattice"> <FINITELATTICE> <LATTICE ref="chain lattice"/> <EXTENT dimension="1" size ="L"/> <BOUNDARY type="open"/> </FINITELATTICE> <UNITCELL ref="simple1d"/> <INHOMOGENEOUS><VERTEX/></INHOMOGENEOUS> </LATTICEGRAPH> </LATTICES>
"Inhomogeneous" tag is the most important tag here. If you set "LATTICEGRAPH" like this you can set the inhomogeneous site term as:
LATTICE_LIBRARY="mylattice.xml" LATTICE="inhomogeneous open chain lattice" MODEL="fermion Hubbard" L=15 t=1 U=-2 mu=-0.001*(x-L/2+0.5)^2
This setting corresponds to the fermions in the quadratic potential.