Dear ALPS users and developers

I want to re-produce the result of a 12x2 spin ladder with four-site interaction.

The four-site interaction is between two nearest-neighboring rungs. 

Please see:

[    Phys. Rev. B 74, 155119 (2006),  http://prb.aps.org/abstract/PRB/v74/i15/e155119     ]

However, it seems that ALPS 2.0 do not support 4-site terms.

Thus I define every rung  as a single site, 

then the ladder becomes a one-dimensional chain (local degree of freedom is 4),

and the four-site interaction becomes a two-site interaction.

In original ladder, the SITEBASIS is just  {sz} = {-1/2, 1/2}.

In this basis operators such as Sminus have very compact expressions

thus they can be simply define as:

  <OPERATOR name="Sminus" matrixelement="sqrt(S*(S+1)-Sz*(Sz-1))">

However, in the new model, a possible choice of SITEBASIS  is {J,Jz}={00, 1-1, 10, 11}.

In this new basis, operators do not have  very compact expressions,

however, the elements of the operator  can be easily obtained  through unitary transformation.

The problem is, I can figure out all the elements of an operator in basis{J, Jz}, but I do not know how to define the operator in "models.xml".

In other words,

suppose an operator X in basis {J, Jz} is explicitly found to be 

[ 1, 0, 0, 0

  0, 2, 1, 0

  0, 1, 3, 0

  0, 0, 0, 4] 

then in ALPS 2.0, is it possible to define X by simply writing down every elements Xij  in "models.xml" ? 

Any suggestion would be appreciated.

sunzhaoyu