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SpinChain

Firstly, we make a lattice for a spin 1/2 chain, 20 sites. The default Hamiltonian will be the isotropic Heisenberg model

spinchain-u1 -L 20 -S 0.5 -o lattice

Now we make a random initial state with quantum number {$S^z=0$}, called 'psi'

mp-random -l lattice -q 0 -o psi

Run the DMRG program for the Heisenberg model, being the operator 'H' in the lattice. Here we are using m=50 states kept and the default is a single-site algorithm with a density matrix mixing term.

mp-dmrg -w psi -H lattice:H -m 50

Repeat this command as needed until it is converged.

The Hamiltonian operator can be more complicated. For example, for a bilinear-biquadratic {$S=1$} chain with {$S=1/2$} edge states at coupling {$\theta=0.3$}, and a reduced coupling strength 0.7J at the edges, use a lattice

spinchain-u1 -L 20 -S 1 --SpinEdge=0.5 -o lattice

with the Hamiltonian as

lattice:"cos(0.3*pi)*(H1Bulk + 0.7*H1Edge) + sin(0.3*pi)*(H2Bulk + 0.7*H2Edge)"

This same procedure can be done with {$SU(2)$} symmetry, by constructing the lattice with

spinchain-su2 L 20 -S 0.5 -o lattice

instead.