muspinsim

MuSpinSim is a Python software meant to simulate muon spectroscopy experiments. In particular, it simulates the spin dynamics of a system of a muon plus other spins, namely electrons and atomic nuclei. It can simulate various common experimental setups and account for hyperfine, dipolar and quadrupolar couplings. It is also able to fit its simulations to experimental data, to find the optimal parameters that describe it.

Installation

You can install the latest release using pip or conda:

pip install muspinsim
conda install muspinsim

Usage

Once installed, the program will be made available for command line use as muspinsim. The usage is

muspinsim input_file.in

where the input file contains the parameters specifying the system and experiment details.

For especially expensive calculations MuSpinSim can also be used in parallel with MPI. In that case, the running command is

mpirun -n <number of cores> muspinsim.mpi input_file.in

where <number of cores> is replaced by the number of desired cores on the given system.

Usage as a library

MuSpinSim can also be used as a Python library within larger programs. The simplest way to do so is to use an input file to configure a problem, read it in with the MuSpinInput class, then use it to create a MuonExperimentalSetup that runs the actual experiment. The minimal script is:

from muspinsim import MuSpinInput, ExperimentRunner

params = MuSpinInput(open('input_file.in'))
experiment = ExperimentRunner(params)

results = experiment.run()

In order to instead run a fitting calculation, the minimal script is

from muspinsim import MuSpinInput, FittingRunner

params = MuSpinInput(open('input_file.in'))
optimizer = FittingRunner(params)

solution = optimizer.run()

For parallel use, it's recommended to stick to using the provided muspinsim.mpi script.

Theory

The way MuSpinSim operates is quite simple, and based on the principles of similar software for NMR, in particular Simpson. A Hamiltonian of the system is built by combining the various terms specified in the input file; hyperfine, dipolar and quadrupolar terms are tied to the orientation of the 'crystallite' of the system we're observing, whereas muon polarization and applied external field are in an absolute reference frame (the one of the laboratory). The density matrix of the system is then made to evolve in time under this Hamiltonian, and the muon polarization is observed by standard quantum mechanical methods to compute expectation values. A slightly different approach is used when saving the integral of the expectation value, as in that case the integration is performed analytically to compute an "integral operator" whose expectation value is then computed. The biggest part of the computational load is tied to the diagonalization of the Hamiltonian, which is currently performed by using the library Numpy. This limits the usefulness of the program right now to matrices smaller than 1000x1000, corresponding roughly to ten spins with I=1/2. Bigger systems might take a while to run or run out of memory on personal computers.

Input files and other information

For more in-depth information about how to use MuSpinSim and about how the theory behind it works, please check the official documentation.