.. _physics-posteriorSampleDffrntlEvltnProposalSize:

Posterior Sampling Differential Evolution Proposal Size
=======================================================

Class providing proposal sizes for differential evolution posterior samplers. Specifically, this class provides the proposal size parameter, :math:`\gamma` (the fraction of the vector connecting to chain state to be used as the proposal for another chain), for use in differential evolution simulations.

Methods
-------

``gamma`` → ``double precision``
   Return the proposal size parameter :math:`\gamma` given the current simulation state and convergence status, scaling the vector difference between two randomly selected chain states to form the differential evolution proposal.

   * ``class(posteriorSampleStateClass ), intent(inout) :: simulationState``

   * ``class(posteriorSampleConvergenceClass), intent(inout) :: simulationConvergence``

.. _physics-posteriorSampleDffrntlEvltnProposalSizeAdaptive:

``posteriorSampleDffrntlEvltnProposalSizeAdaptive``
---------------------------------------------------

This class adaptively changes :math:`\gamma` in an attempt to maintain the acceptance rate at an acceptable level. The algorithm is controlled by the following sub-parameters:

``[gammaInitial]``
   The initial value for :math:`\gamma`.

``[gammaFactor]``
   The multiplicative factor by which :math:`\gamma` should be increased or decreased if the acceptance rate is out of range.

``[gammaMinimum]``
   The smallest value allowed for :math:`\gamma`.

``[gammaMaximum]``
   The largest value allowed for :math:`\gamma`.

``[acceptanceRateMinimum]``
   The minimum acceptance rate to accept before reducing :math:`\gamma`.

``[acceptanceRateMaximum]``
   The maximum acceptance rate to accept before reducing :math:`\gamma`.

``[updateCount]``
   The number of steps between successive checks of the acceptance rate.

**Parameters**

* ``[logFileName]`` (string) — The name of a file to which to log reports of adjustments to :math:`\gamma`. If empty, no reports are logged.
* ``[gammaInitial]`` (real) — The initial value of the proposal scaling parameter :math:`\gamma` used before the acceptance rate has been assessed and any adaptive adjustment has been made.
* ``[gammaMinimum]`` (real) — The minimum value to which the proposal scaling parameter :math:`\gamma` is permitted to be reduced during adaptive adjustment, preventing the step size from becoming vanishingly small.
* ``[gammaMaximum]`` (real) — The maximum value to which the proposal scaling parameter :math:`\gamma` is permitted to be increased during adaptive adjustment, preventing excessively large steps that would degrade acceptance rates.
* ``[gammaAdjustFactor]`` (real) — The multiplicative factor by which :math:`\gamma` is increased or decreased at each adaptation step when the current acceptance rate falls outside the target range.
* ``[acceptanceRateMinimum]`` (real) — The minimum acceptable chain acceptance rate; if the measured acceptance rate falls below this threshold :math:`\gamma` is reduced to produce smaller, more easily accepted proposals.
* ``[acceptanceRateMaximum]`` (real) — The maximum acceptable chain acceptance rate; if the measured acceptance rate exceeds this threshold :math:`\gamma` is increased to produce larger proposals that explore the posterior more efficiently.
* ``[updateCount]`` (integer) — The number of steps between potential updates of the temperature exponent.
* ``[outliersInAcceptanceRate]`` (boolean; default ``.true.``) — The number of steps between potential updates of the proposal size.
* ``[appendLog]`` (boolean; default ``.false.``) — If true, append to the existing log file, otherwise overwrite.
* ``[restoreFromLog]`` (boolean; default ``.false.``) — If true, restore the value of :math:`\gamma` from the log file.
* ``[flushLog]`` (boolean; default ``.false.``) — If true, logs are flushed to file after every update.

.. _physics-posteriorSampleDffrntlEvltnProposalSizeFixed:

``posteriorSampleDffrntlEvltnProposalSizeFixed``
------------------------------------------------

A posterior sampling differential evolution proposal size class in which the proposal size is a fixed value :math:`\gamma=`\ ``[gamma]``.

**Parameters**

* ``[proposalSize]`` (real) — The fixed value of the proposal scaling parameter :math:`\gamma` used to scale the vector difference between two randomly selected chain states when forming differential evolution proposals.