Add a page on the history of the HEOM implementation.

doc/guide/guide-heom.rst

@@ -10,4 +10,5 @@ Hierarchical Equations of Motion
    heom/intro.rst
    heom/bosonic.rst
    heom/fermionic.rst
+   heom/history.rst
    heom/references.rst

doc/guide/heom/history.rst

@@ -0,0 +1,55 @@
+########################
+Previous implementations
+########################
+
+The current HEOM implementation in QuTiP is the latest in a succession of HEOM
+implementations by various contributors:
+
+
+HSolverDL
+---------
+
+The original HEOM solver was implemented by Neill Lambert, Anubhav Vardhan,
+and Alexander Pitchford and is still available as
+:class:`qutip.nonmarkov.dlheom_solver.HSolverDL` and only directly provided
+support for the Drude-Lorentz bath although there was the possibility of
+sub-classing the solver to implement other baths.
+
+A compatible interface using the current implementation is available under the
+same name in :class:`qutip.nonmarkov.heom.HSolverDL`.
+
+
+BoFiN-HEOM
+----------
+
+BoFiN-HEOM (the bosonic and fermionic HEOM solver) was a much more
+flexible re-write of the original QuTiP ``HSolverDL`` that added support for
+both bosonic and fermionic baths and for baths to be specified directly via
+their correlation function expansion coefficients. Its authors were
+Neill Lambert, Tarun Raheja, Shahnawaz Ahmed, and Alexander Pitchford.
+
+BoFiN was written outside of QuTiP and is can still be found in its original
+repository at https://github.com/tehruhn/bofin.
+
+The construction of the right-hand side matrix for BoFiN was slow, so
+BoFiN-fast, a hybrid C++ and Python implementation, was written that performed
+the right-hand side construction in C++. It was otherwise identical to the
+pure Python version. BoFiN-fast can be found at
+https://github.com/tehruhn/bofin_fast.
+
+BoFiN also came with an extensive set of example notebooks that are available
+at https://github.com/tehruhn/bofin/tree/main/examples.
+
+
+Current implementation
+----------------------
+
+The current implementation is a rewrite of BoFiN in pure Python. It's
+right-hand side construction has similar speed to BoFiN-fast, but is written
+in pure Python. Built-in implementations of a variety of different baths
+are provided, and a single solver is used for both fermionic and bosonic baths.
+Multiple baths of the same kind (either fermionic or bosonic) may be
+specified in a single problem, and there is good support for working with
+the auxiliary density operator (ADO) state and extracting information from it.
+
+The code was written by Neill Lambert and Simon Cross.