[ESPResSo-users] Espresso 4.1 released

[Jean-Noël Grad]

Dear Espresso users,

the Espresso team is pleased to announce the release of Espresso 4.1. 
The source can be downloaded at 
https://github.com/espressomd/espresso/releases/download/4.1.0/espresso-4.1.0.tar.gz
Please see the release notes below.


Many thanks to all contributors!
Regards,
Jean-Noël Grad

============

This is a feature release, i.e., new functionality is added to Espresso.
An additional focus of this release is quality assurance and
modernization. The testing of Espresso's functionality has been extended
considerably. Also, sample and tutorial scripts are now automatically
tested. Moreover, a large effort was put into modernizing the C++
simulation core. Work has been done, e.g., on particle sorting, the MPI
communication infrastructure, and the lattice-Boltzmann implementations.
Electrostatic and magnetostatic methods now have a clear and common
interface. These changes will facilitate future extensions of Espresso
and make the code more understandable to new developers.

We recommend that this release be used for all production simulations.
No further bug fix releases will be provided for the 4.0 line, and not
all fixes are present in Espresso 4.0.2.

Please carefully read the detailed list of changes below before using
this release. Issues can be reported at
https://github.com/espressomd/espresso.


Changed requirements
--------------------

* Python 2 support has been dropped. Espresso now requires Python 3.
  For additional information, please see
  https://github.com/espressomd/espresso/wiki/python_2_3_transition.

* Espresso now needs a C++14-capable compiler, such as GCC 4.9 and later
  or Clang 4 and later.

* It is discouraged to use Espresso with Boost versions below 1.67.


Added functionality and documentation
-------------------------------------

* The distance between a shape (such as sphere) and a position can now
  be queried via ``shape.calc_distance()``.

* The lattice nodes of a lattice-Boltzmann fluid can now be iterated
  using ``LBFluid.nodes()``.

* A tutorial on magnetic fluids has been added.

* The stress created by the dissipative particle dynamics interaction
  (DPD) can now be obtained via the ``DPDStress`` observable.

* The stress of a lattice-Boltzmann fluid can now be obtained via the
  ``LBFluidStress`` observable.

* A torus shape has been added.

* Two new accumulators for observables have been added:
  ``MeanVarianceCalculator`` and ``TimeSeries``.

* An ``ElectricPlaneWave`` constraint was added.

* Experimental support for AMD GPUs via HIP. The future of this feature
  is unclear. Please do not base hardware buying decisions on its
  presence.

* Visualization of slit pores in the OpenGL visualizer.

* A Weeks-Chandler-Anderson short-range potential has been added
  (``WCA``).

* The external force density applied to a lattice-Boltzmann fluid
  can now be changed during the simulation.

* Sanity checks for Mach limits and unequal MD and lattice-Boltzmann
  time steps have been added.

* The ``system.cell_system.tune_skin()`` method now has a keyword
  argument ``adjust_max_skin``. If set to ``True``, the maximum skin
  to be tested will be reduced such that it is compatible with the local
  box size.

* For the CPU lattice-Boltzmann implementation, the limit on the Verlet
  list skin (<0.5 agrid) has been lifted.

* A new observable ``CosPersistenceAngles`` has been added for the bond
  angles of a polymer (needed, e.g., for determining the persistence
  length).


Feature configuration at compile time
-------------------------------------

* The number of features which need to be defined at compile time in
  myconfig.hpp has been reduced. Features without performance impact
  are now always present. These are:

  * ``PARTIAL_PERIODIC``
  * ``LB``
  * ``LB_GPU`` (for builds with CUDA)
  * ``IMMERSED_BOUNDARY``
  * ``BOND_ANGLE``

* For most compilers, it is checked that only known features are
  declared in myconfig.hpp.

* The default feature configuration applied when no myconfig.hpp is
  present has been extended significantly. In particular, all tutorials
  can now be run with the default feature configuration. For production
  simulations, it is still recommended to use a custom myconfig.hpp
  containing only necessary features. This is true in particular, if
  particle rotation is not needed.

* Features that do not have automated tests now require
  ``EXPERIMENTAL_FEATURES`` to be defined in myconfig.hpp.


Interface changes
-----------------

* Several parts of Espresso now use a method-specific seed for random
  number generation. For the following individual methods a random
  number seed has to be passed using the keyword argument ``seed``:

   * [Langevin 
thermostat](http://espressomd.org/html/doc/espressomd.html?#espressomd.thermostat.Thermostat.set_langevin)
   * [DPD 
thermostat](http://espressomd.org/html/doc/espressomd.html?#espressomd.thermostat.Thermostat.set_dpd)
   * [LB 
Thermostat](http://espressomd.org/html/doc/espressomd.html?#espressomd.thermostat.Thermostat.set_lb)
   * [Thermalized 
bond](http://espressomd.org/html/doc/espressomd.html?#espressomd.interactions.ThermalizedBond)
   * [Wang-Landau reaction 
ensemble](http://espressomd.org/html/doc/espressomd.html?#module-espressomd.reaction_ensemble)
   * [Constant pH 
ensemble](http://espressomd.org/html/doc/espressomd.html?#espressomd.reaction_ensemble.ConstantpHEnsemble)
   * [Widom insertion 
method](http://espressomd.org/html/doc/espressomd.html?#espressomd.reaction_ensemble.WidomInsertion)

* Changes in the lattice-Boltzmann (LB) interface:

   * By default the LB fluid is not thermalized. A temperature can be
     set using the ``LBFluid``'s keyword parameter ``kT``. If
     ``kT > 0``, an additional ``seed`` keyword parameter has to be
     provided.
   * The LB thermostat gets its temperature from the LB fluid.
   * The frictional coupling coefficient ``gamma`` is now a keyword
     parameter of the LB thermostat.

  For more detailed information on how to set up a LB fluid and
  thermostat, please see http://espressomd.org/html/doc/lb.html.

* The method for polymer creation has been replaced. Now
  ``espressomd.polymer.positions()`` can be used to obtain particle
  positions for one or more polymer chains. Based on these positions,
  polymers can be created. For an example please see
  http://espressomd.org/html/doc/particles.html#setting-up-polymer-chains.


Changed and removed functionality
---------------------------------

* The ``remove_total_momentum()`` method for lattice-Boltzmann fluids
  has been removed. The overall velocity of a fluid can be changed
  using the ``lb_fluid.nodes()`` iterator.

* The ``CATALYTIC_REACTIONS`` feature has been removed.

* The method for creating a polymer has been replaced.
  ``espressomd.polymer.positions()`` can now be used to obtain particle
  positions for one or more polymer chains.

* Checkpointing has been added for the electrokinetics method.

* The global random number seed has been partly replaced by
  method-specific ones. These are specified when activating the relevant
  feature such as the Langevin, DPD and lattice-Boltzmann thermostats
  via a ``seed`` keyword argument.

* The random number generator has been switched to Philox for most
  algorithms requiring random numbers.

* Limitations on the exclusion radius have been relaxed in the reaction
  ensemble method.

* A new observable ``CosPersistenceAngles`` has been added for the bond
  angles of a polymer (needed, e.g., for determining the persistence
  length).

* ELC has been disabled for non-neutral systems with constant potential.

* The calculation of the linear particle momentum included the forces of
  the last time step. The function ``system.analysis.linear_momentum()``
  now returns the sum of the product of mass and velocity of all
  particles, if no lattice-Boltzmann fluid is coupled.


Performance enhancements
------------------------

* Speedup in the short-range force calculation in situations where the
  short-range cutoff varies strongly for different pairs of particles,
  e.g., in a bidisperse fluid.

* Speedup in particle resorting triggered when particles have moved by
  more than a skin.

* Significantly faster back-transfer of particle forces from GPU-based
  methods such as the GPU implementations of lattice-Boltzmann and P3M.


Bug fixes
---------

* Lattice-Boltzmann boundaries, constraints and auto_update_accumulators
  are now included in checkpointing. (#2915)

* Collision detection is now checkpointed. (#2342)

* The rhomboid shape was fixed. (#2756)

* Deadlocks on certain GPUs have been resolved for the dipolar
  Barnes-Hut method. (#2719)

* The visualization of dihedrals has been fixed. (#2677)

* The ``ENGINE`` implementation for CPU LB has been fixed. (#3025)

* The external force density in lattice-Boltzmann fluids is no longer
  ignored in the first integration step after setting the force density.
  (#3144)

* The positions of virtual sites and the charges of ICC particles are
  now updated before observable calculation. (#3128)

* The forces and torques for the Gay-Berne potential have been
  corrected. (#3091)

* Remove undocumented behaviour in the case of using a cylindrical
  sampling area in the reaction ensemble, constant pH ensemble,
  Wang-Landau ensemble, Widom-Insertion method. (#3174)

* The ELC tuning error calculation has been rearranged to produce
  correct results for higher accuracies. (#3123)


New tutorials
-------------

* Tutorials for simulating ferrofluids and for using the constant-pH
  method have been added.


Under the hood changes
----------------------

* Automated testing has been enhanced. It now also includes samples and
  tutorials. The overall test coverage for the simulation core has
  increased by ~12% since Espresso 4.0.2.

* The CPU LB and LB-particle coupling have been refactored.

* Particle resorting has been simplified and sped-up.

* The MPI callback mechanism has been simplified. Furthermore, reduction
  operations such as summing values from all MPI ranks can now be
  performed.

* Nearly all manual memory management and C-style arrays have been
  removed.

* The rotation-related code has been simplified.

* Long-range electrostatic and magnetostatic methods now have a common
  interface.

* The kernels for short-range and bonded interactions have been
  simplified.

* The CMake build system has been refactored and dependencies between
  different parts of the code have been made clear.

* Python code formatting: the autopep8 version now matches the one in
  Ubuntu 18.04 (autopep8 v1.3.4 with pycodestyle v2.3.1).