Previously, I used DMRG to calculate the model containing the external potential, but now I would like to try to use MPS to calculate it.When i use MPS, I meet a question of  potential.

My potential setting  like this

<SITETERM site="i">

<PARAMETER name="mu#" default="mu"/>

<PARAMETER name="U#" default="U"/>

<PARAMETER name="Vout" default="0"/>

-mu#*n(i)+U#*n_up(i)*n_down(i)+Vout*((x-(L-1)/2.0)^2)*n(i)

</SITETERM>

I use the model ,lattice and command like I used in DMRG, but the computer prompt like :

Exception thrown:

Cannot evaluate x

By using DMRG, it can be calculated correctly, but MPS will prompt the error, i wonder what should I do to correct it?

It is highly grateful for your any suggestion.

Yours sincerely,

Xingbo Wei

Zhejiang Normal University

Jinhua,230014,

China