========

evoMPS |. |.:|::>

Tutorial videos:

• Installation <http://vimeo.com/user19042101/evomps-tutorial-installation>
• Find ground state <http://vimeo.com/user19042101/evomps-tutorial-ground>
• Find excitations <http://vimeo.com/user19042101/evomps-tutorial-excitations>

evoMPS simulates time-evolution (real or imaginary) of one-dimensional many-particle quantum systems using matrix product states (MPS) and the time dependent variational principle (TDVP).

It can be used to efficiently find ground states and simulate dynamics.

The evoMPS implementation assumes a nearest-neighbour or next-nearest-neighbour Hamiltonian and one of the following situations:

• states on a finite chain with open boundary conditions
• spatially uniform states on an infinite chain
• otherwise uniform states with a localized nonuniformity on an infinite chain (only nearest-neighbour at present)

It is based on algorithms published by:

• Jutho Haegeman
• J. Ignacio Cirac
• Tobias J. Osborne
• Iztok Pizorn
• Henri Verschelde
• Frank Verstraete

and available on arxiv.org under arXiv:1103.0936v2 [cond-mat.str-el] <http://arxiv.org/abs/1103.0936v2>. The algorithm for handling localized nonuniformities on infinite chains was developed by:

• Ashley Milsted
• Tobias J. Osborne
• Frank Verstraete
• Jutho Haegeman

and is detailed in arXiv:1207.0691 <http://arxiv.org/abs/1207.0691>. For details, see doc/implementation_details.pdf and the source code itself, which I endeavour to annotate thoroughly.

evoMPS is implemented in Python using Scipy <http://www.scipy.org> and benefits from optimized linear algebra libraries being installed (BLAS and LAPACK). For more details, see INSTALL.

evoMPS was originally developed as part of an MSc project by Ashley Milsted, supervised by Tobias Osborne at the Institute for Theoretical Physics of Leibniz Universität Hannover <http://www.itp.uni-hannover.de/>.

The evoMPS algorithms are presented as python classes to be used in a script. Some example scripts can be found in the "examples" directory. To run an example script without installing the evoMPS modules, copy it to the base directory first e.g. under Windows:

copy examples\transverse_ising_uniform.py .
python transverse_ising_uniform.py

Essentially, the user defines a spin chain Hilbert space and a nearest-neighbour Hamiltonian and then carries out a series of small time steps (numerically integrating the "Schrödinger equation" for the MPS parameters):

sim = EvoMPS_TDVP_Uniform(bond_dim, local_hilb_dim, my_hamiltonian)

for i in range(max_steps):
    sim.update()

    my_exp_val = sim.expect_1s(my_op)

    sim.take_step_RK4(dtau)

Operators, including the Hamiltonian, are defined as arrays like this:

pauli_z = numpy.array([[1, 0],
                       [0, -1]])

or as python callables (functions) like this:

def pauli_z(s, t):
    if s == t:
        return (-1.0)**s
    else:
        return 0

Calculating expectation values or other quantities can be done after each step as desired.

Switching between imaginary time evolution (for finding the ground state) and real time evolution is as easy as multiplying the time step size by a factor of i!

Please send comments to:

ashmilsted at <google's well-known email service>

To submit ideas or bug reports, please use the GitHub Issues system <http://github.com/amilsted/evoMPS/>.