This is a repository for the *Google Summer of Code* project on Differentiable Tensor Networks.

In this package we implemented the algorithms described in Differentiable Programming Tensor Networks, namely implementing *automatic differentiation* (AD) on *Corner Transfer Matrix Renormalization Group* (CTMRG) and *Tensor Renormalization Group* (TRG),
demonstrating two applications:

- Gradient based optimization of iPEPS
- Direct calculation of energy densities in iPEPS via derivatives of the
*free energy*

More generally we aim to provide Julia with the tools to combine AD and tensor network methods.

Suggestions and Comments in the *Issues* are welcome.

## Example

Since this package was inspired by the Differentiable Programming Tensor Networks paper, we demonstrate the usage of those algorithms in the following.

### Free Energy of the 2D Classical Ising Model

We start by constructing the tensor for the tensor network representation of the 2d classical Ising Model.
This tensor can be constructed using the `model_tensor`

-function that takes a `model`

-parameter - in our case `Ising()`

- and an inverse temperature `β`

(e.g. at `β=0.5`

).

```
julia> a = model_tensor(Ising(), 0.5)
2×2×2×2 Array{Float64,4}:
[:, :, 1, 1] =
2.53434 0.5
0.5 0.18394
[:, :, 2, 1] =
0.5 0.18394
0.18394 0.5
[:, :, 1, 2] =
0.5 0.18394
0.18394 0.5
[:, :, 2, 2] =
0.18394 0.5
0.5 2.53434
```

Using the `trg`

function, we can calculate the partition function of the model per site:

```
julia> trg(a, 20,20)
1.0257933734351765
```

which grows bond-dimensions up to `20`

and does `20`

iterations, i.e. grows the system to a size of `2^20`

which is well converged for our purposes.

Given the partition function, we get the free energy as the first derivative with respect to `β`

times `-1`

.
With Zygote, this is straightforward to calculate:

```
julia> using Zygote: gradient
julia> dβ = gradient(β -> trg(model_tensor(Ising(), β), 20,20), 0.5)[1]
1.7455677143228514
julia> -dβ
-1.7455677143228514
```

which agrees with the data presented in the paper.

### Finding the Ground State of infinite 2D Heisenberg model

The other algorithm variationally minimizes the energy of a Heisenberg model on a two-dimensional infinite lattice using a form of gradient descent.

First, we need the hamiltonian as a tensor network operator

```
julia> h = hamiltonian(Heisenberg())
2×2×2×2 Array{Float64,4}:
[:, :, 1, 1] =
-0.5 0.0
0.0 0.5
[:, :, 2, 1] =
0.0 0.0
-1.0 0.0
[:, :, 1, 2] =
0.0 -1.0
0.0 0.0
[:, :, 2, 2] =
0.5 0.0
0.0 -0.5
```

where we get the `Heisenberg`

-hamiltonian with default parameters `Jx = Jy = Jz = 1.0`

.
Next we initialize an ipeps-tensor and calculate the energy of that tensor and the hamiltonian:

```
julia> ipeps = SquareIPEPS(rand(2,2,2,2,2));
julia> ipeps = TensorNetworkAD.indexperm_symmetrize(ipeps);
julia> TensorNetworkAD.energy(h,ipeps, χ=20, tol=1e-6,maxit=100)
-0.5278485155836766
```

where the initial energy is random.

To minimise it, we combine `Optim`

and `Zygote`

under the hood to provide the `optimiseipeps`

function.

```
julia> using Optim
julia> res = optimiseipeps(ipeps, h; χ=20, tol=1e-6, maxit=100,
optimargs = (Optim.Options(f_tol=1e-6, show_trace=true),));
Iter Function value Gradient norm
0 -5.015158e-01 2.563357e-02
* time: 4.100799560546875e-5
1 -6.171409e-01 3.170732e-02
* time: 0.3943500518798828
2 -6.558814e-01 2.927539e-02
* time: 0.6722378730773926
3 -6.577320e-01 1.299056e-02
* time: 1.0529990196228027
4 -6.587514e-01 8.515789e-03
* time: 1.2889769077301025
5 -6.595896e-01 1.102446e-02
* time: 1.5059330463409424
6 -6.599735e-01 2.020418e-03
* time: 1.8917429447174072
7 -6.600449e-01 4.343536e-03
* time: 2.180701971054077
8 -6.601202e-01 2.623793e-03
* time: 2.5907390117645264
9 -6.602188e-01 3.951503e-04
* time: 2.9895379543304443
10 -6.602232e-01 2.597750e-04
* time: 3.254667043685913
11 -6.602246e-01 2.960359e-04
* time: 3.4899749755859375
12 -6.602282e-01 2.846450e-04
* time: 3.739893913269043
13 -6.602290e-01 1.679273e-04
* time: 3.9142658710479736
14 -6.602303e-01 2.155790e-04
* time: 4.230381011962891
15 -6.602311e-01 2.239934e-05
* time: 4.5699989795684814
16 -6.602311e-01 1.935087e-05
* time: 4.837096929550171
```

where our final value for the energy `e = -0.6602`

agrees with the value found in the paper.

## License

MIT License