# Setting Options for the Dynamics Solvers

Occasionally it is necessary to change the built in parameters of the dynamics solvers used by for example the `qutip.mesolve` and `qutip.mcsolve` functions. The options for all dynamics solvers may be changed by using the Options class `qutip.solver.Options`.

    options = Options()

the properties and default values of this class can be view via the print function:

    print(options)

Output:

    Options:
    -----------
    atol:              1e-08
    rtol:              1e-06
    method:            adams
    order:             12
    nsteps:            1000
    first_step:        0
    min_step:          0
    max_step:          0
    tidy:              True
    num_cpus:          2
    norm_tol:          0.001
    norm_steps:        5
    rhs_filename:      None
    rhs_reuse:         False
    seeds:             0
    rhs_with_state:    False
    average_expect:    True
    average_states:    False
    ntraj:             500
    store_states:      False
    store_final_state: False

These properties are detailed in the following table. Assuming `options = Options()`:

Property | Default setting | Description
--- | --- | ---
options.atol | 1e-8 | Absolute tolerance
options.rtol | 1e-6 | Relative tolerance
options.method | ‘adams’ | Solver method. Can be ‘adams’ (non-stiff) or ‘bdf’ (stiff)
options.order | 12 | Order of solver. Must be <=12 for ‘adams’ and <=5 for ‘bdf’
options.nsteps | 1000 | Max. number of steps to take for each interval
options.first_step | 0 | Size of initial step. 0 = determined automatically by solver.
options.min_step | 0 | Minimum step size. 0 = determined automatically by solver.
options.max_step | 0 | Maximum step size. 0 = determined automatically by solver.
options.tidy | True | Whether to run tidyup function on time-independent Hamiltonian.
options.store_final_state | False | Whether or not to store the final state of the evolution.
options.store_states | False | Whether or not to store the state vectors or density matrices.
options.rhs_filename | None | RHS filename when using compiled time-dependent Hamiltonians.
options.rhs_reuse | False | Reuse compiled RHS function. Useful for repetitive tasks.
options.rhs_with_state | False | Whether or not to include the state in the Hamiltonian function callback signature.
options.num_cpus | installed num of processors | Integer number of cpus used by mcsolve.
options.seeds | None | Array containing random number seeds for mcsolver.
options.norm_tol | 1e-6 | Tolerance used when finding wavefunction norm in mcsolve.
options.norm_steps | 5 | Max. number of steps used to find wavefunction’s norm to within norm_tol in mcsolve.
options.steady_state_average | False | Include an estimation of the steady state in mcsolve.
options.ntraj | 500 | Number of trajectories in stochastic solvers.
options.average_expect | True | Average expectation values over trajectories.
options.average_states | False | Average of the states over trajectories.
options.openmp_threads | installed num of processors | Number of OPENMP threads to use.
options.use_openmp | None | Use OPENMP for sparse matrix vector multiplication.

As an example, let us consider changing the number of processors used, turn the GUI off, and strengthen the absolute tolerance. There are two equivalent ways to do this using the Options class. First way,

    options = Options()
    options.num_cpus = 3
    options.atol = 1e-10

or one can use an inline method,

    options = Options(num_cpus=4, atol=1e-10)

Note that the order in which you input the options does not matter. Using either method, the resulting options variable is now:

    print(options)

Output:

    Options:
    -----------
    atol:              1e-10
    rtol:              1e-06
    method:            adams
    order:             12
    nsteps:            1000
    first_step:        0
    min_step:          0
    max_step:          0
    tidy:              True
    num_cpus:          4
    norm_tol:          0.001
    norm_steps:        5
    rhs_filename:      None
    rhs_reuse:         False
    seeds:             0
    rhs_with_state:    False
    average_expect:    True
    average_states:    False
    ntraj:             500
    store_states:      False
    store_final_state: False

To use these new settings we can use the keyword argument `options` in either the func:qutip.mesolve and `qutip.mcsolve` function. We can modify the last example as:

    >>> mesolve(H0, psi0, tlist, c_op_list, [sigmaz()], options=options)
    >>> mesolve(hamiltonian_t, psi0, tlist, c_op_list, [sigmaz()], H_args, options=options)

or:

    >>> mcsolve(H0, psi0, tlist, ntraj,c_op_list, [sigmaz()], options=options)
    >>> mcsolve(hamiltonian_t, psi0, tlist, ntraj, c_op_list, [sigmaz()], H_args, options=options)