Defining a New Operator when Inheriting CouplingMPOModel

[stephenjnaus]

I'm new at TeNPy and am trying to create a model that is a CouplingMPOModel, with a new operator Nj.

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class myTFLIMModel(CouplingMPOModel):
    def init_sites(self, model_params):
        site = SpinHalfSite(conserve=None)
        Nj = np.array([[1,0],[0,0]])
        site.add_op('Nj', Nj)
        return site
    
    def init_terms(self, model_params):
    	U = model_params.get('U',1.)
        self.add_coupling(U, 0, 'Nj',1,'Nj',np.array([0]))
 

However, when I try to create an object of this class by doing the following:

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model_params = dict(lattice = Ladder(L = 20, site = SpinHalfSite(conserve=None)), L=20, U=1.,bc_MPS='finite', conserve='None') 
M = myTFLIMModel(model_params)

I keep receiving an error that claims it has no knowledge of the Nj operator. In particular it says:

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"ValueError: unknown onsite operator 'Nj' for u=0
SpinHalfSite('None')"

I don't understand why Nj has not been recognized. Interestingly, I have run a similar code without specifying a particular lattice and it has run without any errors, so I'm assuming something went wrong there. Any help would be much appreciated.

[Johannes]

The issue here is that you directly pass the full Ladder instance in the model_params, which already has the site defined - hence the model doesn't call init_sites anymore.
Try to use

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model_params = dict(
    lattice='Ladder',
    L=20,
    U=1.,
    bc_MPS='finite'
)

and overwrite init_lattice if you need to adjust creating the lattice in a specialized way.