## Learn to Use ITensor

# DMRG Interface

The density matrix renormalization group (DMRG) is a very powerful algorithm for optimizing matrix product states (MPS) for ground states and low-lying excited states of one-dimensional (and quasi-one-dimensional) Hamiltonians.

The ITensor DMRG code works with Hamiltonians in matrix product operator (MPO) form so that a single DMRG code can work for a very wide range of different Hamiltonians and local Hilbert spaces.

ITensor also offers DMRG routines for more general optimization tasks, including

- Hamiltonians which are an (implicit) sum of multiple MPOs
- minimizing the energy of an MPS, while constraining it to remain orthogonal to a collection of other MPS (for targeting excited states)
- "restricted" sweeping over a sub-region of a system (such as a unit cell) within an environment defined by a "frozen" MPS basis for the rest of the system

## Synopsis

```
int N = 100;
auto sites = SpinOne(N);
auto ampo = AutoMPO(sites);
for(int j = 1; j < N; ++j)
{
ampo += "Sz",j,"Sz",j+1;
ampo += 0.5,"S+",j,"S-",j+1;
ampo += 0.5,"S-",j,"S+",j+1;
}
auto H = toMPO(ampo);
auto sweeps = Sweeps(5);
sweeps.maxdim() = 10,40,100,200,200;
sweeps.cutoff() = 1E-8;
// Create a random starting state
auto state = InitState(sites);
for(auto i : range1(N))
{
if(i%2 == 1) state.set(i,"Up");
else state.set(i,"Dn");
}
auto psi0 = randomMPS(state);
auto [energy,psi] = dmrg(H,psi0,sweeps,{"Quiet",true});
printfln("Ground state energy = %.20f",energy);
```

## Basic DMRG interface

dmrg(MPO H, MPS psi0, Sweeps sweeps, Args args = Args::global()) -> std::tuple<Real,MPS>

Run a DMRG calculation to find the ground state of the Hamiltonian (MPO) H using the initial state given by the MPS psi0. A tuple of the final ground state energy and the optimized MPS ground state approximation is returned.

The number of sweeps and DMRG accuracy parameters are controlled by a Sweeps object.

After the DMRG calculation completes, it returns the final ground state energy.

The

`dmrg`

function also accepts the following optional named arguments:- "Quiet" — suppress most output except a short summary of the result of each DMRG sweep
- "WriteDim" — if WriteDim is defined, upon beginning a sweep number n such that sweeps.maxdim(n) value exceeds WriteDim, the MPS and MPO will be set to "write-to-disk" mode such that all but a few "core" tensors will remain on the hard drive to save on RAM
- "DebugLevel" — non-negative integer telling the internal Davidson eigensolver how much extra information to print out
- "Silent" — suppress all output of DMRG and perform no measurements

## Generalized DMRG interfaces

dmrg(std::vector<MPO> H, MPS psi0, Sweeps sweeps, Args args = Args::global()) -> std::tuple<Real,MPS>

Run a DMRG calculation using a collection of MPOs provided in a std::vector object (0-indexed).

The MPOs will be treated as if they are summed. That is, the Hamiltonian is defined as the sum of the MPOs provided. However, no actual sum of the MPOs will be performed, such that the cost of the algorithm is the sum of the costs of using each MPO individually.

dmrg(MPO H, std::vector<MPS> wfs, MPS psi0, Sweeps sweeps, Args args = Args::global()) -> std::tuple<Real,MPS>

Run a DMRG calculation to find the lowest energy eigenstate of the Hamiltonian H, but with the constraint that psi should be orthogonal to all of the MPS provided in the (0-indexed) std::vector wfs.

This code works by putting in an energy penalty for psi to have any overlap with the MPS in wfs. This penalty is the product of the overlap of psi with each state times a "Weight", whose default value is 1.0 but which can be adjusted by providing the named argument "Weight" as part of the args (last argument of type Args).

To see a detailed example of using this excited-state DMRG code, see the code formula on computing excited states.

dmrg(MPO H, ITensor LH, ITensor RH MPS psi0, Sweeps sweeps, Args args = Args::global()) -> std::tuple<Real,MPS>

Run a DMRG calculation to find the ground state of the Hamiltonian H using the initial state psi0, using a "boundary environment" defined by the tensors LH and RH.

The boundary environments LH and RH (e.g. representing the projection of a Hamiltonian for a larger system into a fixed "MPS basis") should be tensors with three indices. One index connects to the bond index of the Hamiltonian MPO H at the left and right edge. The other two indices are i and dag(i') where i is the virtual (bond) index of the MPS psi at the left or right edge (and dag(i') is i with prime level 1 and reversed arrow in the case of an Index).

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