# 1.6. How to use Expert mode¶

If you prepare input files, you can perform calculations for arbitrary Hamiltonians with any one-body potentials and the two-body interactions. By taking spin 1/2 system as an example, we explain how to prepare input files. For spin 1/2 system, we prepare simple python scripts (samples/tutorial_1.6/MakeDef.py) that can generate the input files for general Hamiltonians, which are defined as

(1.9)${\mathcal H}=\sum_{i,j} J_{i,j}^{\alpha,\beta} {\bf S}_{i}^{\alpha} {\bf S}_{j}^{\beta}.$

Note tat samples/tutorial_1.6/read.py and samples/tutorial_1.6/hphi_io.py are necessary for MakeDef.py. To use MakeDef.py, it is necessary to prepare two input files, input.txt and pair.txt (examples of them are available in samples/tutorial_1.6 ).

In input.txt, two parameters Ns (number of sites) and exct (number of excited states) are specified.

Below is an example of input.txt for 2 site Heisenberg model

Ns 2
exct 2


In pair.txt, one specify the interaction terms in the form

(1.10)$i~~~~~j~~~~~\alpha~~~~~\beta~~~~~J_{i,j}^{\alpha,\beta}$

For diagonal interactions ($$\alpha=\beta$$), $$J_{i,j}^{\alpha,\alpha} {\bf S}_{i}^{\alpha} {\bf S}_{j}^{\alpha}$$ is added, and $$J_{i,j}^{\alpha,\beta} [{\bf S}_{i}^{\alpha} {\bf S}_{j}^{\beta}+{\bf S}_{j}^{\alpha} {\bf S}_{i}^{\beta} ]$$ is added for off-diagonal interactions ($$\alpha\neq\beta$$),

Below is an example of pair.txt for 2 site Heisenberg model

0 1 x x 0.5
0 1 y y 0.5
0 1 z z 0.5


One can also specify the off-diagonal interaction as

0 1 x x 0.5
0 1 y y 0.5
0 1 z z 0.5
0 1 x y 0.5
0 1 x z 0.5
0 1 y z 0.5


Note that interaction $$(\alpha,\beta)$$ and $$(\beta,\alpha)$$ generate the same interaction since we assume $$J_{i,j}^{\alpha,\beta}$$ is real.

## 1.6.1. Exercise¶

As a simple exercise, for small system sizes (e.g. 2-site system), please try to confirm that $$H_{xy} = \sum_{i,j} J_{i,j}^{x,y} [{\bf S}_{i}^{x} {\bf S}_{j}^{y}+{\bf S}_{j}^{x} {\bf S}_{i}^{y} ]$$, $$H_{yz} = \sum_{i,j} J_{i,j}^{y,z} [{\bf S}_{i}^{y} {\bf S}_{j}^{z}+{\bf S}_{j}^{y} {\bf S}_{i}^{z} ]$$, and $$H_{zx} = \sum_{i,j} J_{i,j}^{z,x} [{\bf S}_{i}^{z} {\bf S}_{j}^{x}+{\bf S}_{j}^{z} {\bf S}_{i}^{x} ]$$ are equivalent by calculating the ground-state energy and the excited-state energies.

As an advanced exercise, please try to make input files for the Kitaev model on the honeycomb lattice by changing pair.txt and input.txt. Since the Kitaev model can also be treated in Standard mode, please confirm the results by Expert mode are consistent with those by Standard mode.

Another example is the XY model on the one-dimensional chain. Although Standard mode only supports the Heisenberg model, it is easy to treat XY model by omitting “CoulombInter coulombinter.def” and “Hund hund.def” in namelist.def generated for the Heisenberg model. Please confirm the results by Expert mode are consistent with those by Standard mode.