[pull] develop from lammps:develop

doc/doxygen/Doxyfile.in

@@ -2,7 +2,7 @@
 
 DOXYFILE_ENCODING      = UTF-8
 PROJECT_NAME           = "LAMMPS Programmer's Guide"
-PROJECT_NUMBER         = "11 February 2026"
+PROJECT_NUMBER         = "30 March 2026"
 PROJECT_BRIEF          = "Documentation of the LAMMPS library interface and Python wrapper"
 PROJECT_LOGO           = lammps-logo.png
 CREATE_SUBDIRS         = NO

doc/lammps.1

@@ -1,7 +1,7 @@
-.TH LAMMPS "1" "11 February 2026" "2026-02-11"
+.TH LAMMPS "1" "30 March 2026" "2026-03-30"
 .SH NAME
 .B LAMMPS
-\- Molecular Dynamics Simulator.  Version 11 February 2026
+\- Molecular Dynamics Simulator.  Version 30 March 2026
 
 .SH SYNOPSIS
 .B lmp

doc/src/Build_extras.rst

@@ -1258,7 +1258,7 @@ then load this plugin at runtime with the :doc:`plugin command
 
    .. tab:: CMake build
 
-      .. versionchanged:: TBD
+      .. versionchanged:: 30Mar2026
 
          Replaced MD5 checksums with SHA-256
 
@@ -1300,7 +1300,7 @@ folder and then load this plugin at runtime with the :doc:`plugin command <plugi
 
    .. tab:: CMake build
 
-      .. versionchanged:: TBD
+      .. versionchanged:: 30Mar2026
 
          Replaced MD5 checksums with SHA-256
 

doc/src/Commands_bond.rst

@@ -74,6 +74,7 @@ OPT.
    * :doc:`charmm (iko) <angle_charmm>`
    * :doc:`class2 (ko) <angle_class2>`
    * :doc:`class2/p6 <angle_class2>`
+   * :doc:`class2xe <angle_class2>`
    * :doc:`cosine (ko) <angle_cosine>`
    * :doc:`cosine/buck6d <angle_cosine_buck6d>`
    * :doc:`cosine/delta (o) <angle_cosine_delta>`
@@ -122,6 +123,7 @@ OPT.
    * :doc:`charmm (iko) <dihedral_charmm>`
    * :doc:`charmmfsw (k) <dihedral_charmm>`
    * :doc:`class2 (ko) <dihedral_class2>`
+   * :doc:`class2xe <dihedral_class2>`
    * :doc:`cosine/shift/exp (o) <dihedral_cosine_shift_exp>`
    * :doc:`cosine/squared/restricted <dihedral_cosine_squared_restricted>`
    * :doc:`fourier (iko) <dihedral_fourier>`

doc/src/Howto_bioFF.rst

@@ -1,5 +1,5 @@
-CHARMM, AMBER, COMPASS, DREIDING, and OPLS force fields
-=======================================================
+CHARMM, AMBER, COMPASS, ClassII-xe, DREIDING, and OPLS force fields
+===================================================================
 
 Here we only discuss formulas implemented in LAMMPS that correspond to
 formulas commonly used in the CHARMM, AMBER, COMPASS, and DREIDING force
@@ -50,17 +50,17 @@ The interaction styles listed below compute force field formulas that
 are consistent with common options in CHARMM or AMBER.  See each
 command's documentation for the formula it computes.
 
-* :doc:`bond_style <bond_harmonic>` harmonic
-* :doc:`angle_style <angle_charmm>` charmm
-* :doc:`dihedral_style <dihedral_charmm>` charmmfsh
-* :doc:`dihedral_style <dihedral_charmm>` charmm
-* :doc:`pair_style <pair_charmm>` lj/charmmfsw/coul/charmmfsh
-* :doc:`pair_style <pair_charmm>` lj/charmmfsw/coul/long
-* :doc:`pair_style <pair_charmm>` lj/charmm/coul/charmm
-* :doc:`pair_style <pair_charmm>` lj/charmm/coul/charmm/implicit
-* :doc:`pair_style <pair_charmm>` lj/charmm/coul/long
-* :doc:`special_bonds <special_bonds>` charmm
-* :doc:`special_bonds <special_bonds>` amber
+* :doc:`bond_style harmonic <bond_harmonic>`
+* :doc:`angle_style charmm <angle_charmm>`
+* :doc:`dihedral_style charmmfsh <dihedral_charmm>`
+* :doc:`dihedral_style charmm <dihedral_charmm>`
+* :doc:`pair_style lj/charmmfsw/coul/charmmfsh <pair_charmm>`
+* :doc:`pair_style lj/charmmfsw/coul/long <pair_charmm>`
+* :doc:`pair_style lj/charmm/coul/charmm <pair_charmm>`
+* :doc:`pair_style lj/charmm/coul/charmm/implicit <pair_charmm>`
+* :doc:`pair_style lj/charmm/coul/long <pair_charmm>`
+* :doc:`special_bonds charmm <special_bonds>`
+* :doc:`special_bonds amber <special_bonds>`
 
 The pair styles compute Lennard Jones (LJ) and Coulombic interactions
 with additional switching or shifting functions that ramp the energy
@@ -174,16 +174,104 @@ These interaction styles listed below compute force field formulas that
 are consistent with the COMPASS force field.  See each command's
 documentation for the formula it computes.
 
-* :doc:`bond_style <bond_class2>` class2
-* :doc:`angle_style <angle_class2>` class2
-* :doc:`dihedral_style <dihedral_class2>` class2
-* :doc:`improper_style <improper_class2>` class2
+* :doc:`bond_style class2 <bond_class2>`
+* :doc:`angle_style class2 <angle_class2>`
+* :doc:`dihedral_style class2 <dihedral_class2>`
+* :doc:`improper_style class2 <improper_class2>`
 
-* :doc:`pair_style <pair_class2>` lj/class2
-* :doc:`pair_style <pair_class2>` lj/class2/coul/cut
-* :doc:`pair_style <pair_class2>` lj/class2/coul/long
+* :doc:`pair_style lj/class2 <pair_class2>`
+* :doc:`pair_style lj/class2/coul/cut <pair_class2>`
+* :doc:`pair_style  lj/class2/coul/long<pair_class2>`
 
-* :doc:`special_bonds <special_bonds>` lj/coul 0 0 1
+* :doc:`special_bonds lj/coul 0 0 1 <special_bonds>`
+
+ClassII-xe
+----------
+
+.. versionadded:: 30Mar2026
+
+The computationally efficient simulation of condensed-phase materials
+such as metals, polymers, and composites can be achieved with fixed-bond
+force fields.  Two popular fixed-bond force fields for such materials
+are COMPASS :ref:`(Sun) <howto-Sun>` and PCFF :ref:`(Maple)
+<howto-Maple>`.  Both use a Class II-based functional form that
+incorporates anharmonic vibrational modes via the quartic Taylor-series
+expansion of the Hamiltonian.  In addition, Class II-based force fields
+add intermolecular coupling via cross-term interactions that also
+account for more complex vibrational modes.  The underlying harmonic
+bonds and harmonic cross-terms can limit the reliability of the physics,
+e.g., when calculating mechanical properties.  To overcome the harmonic
+bonding limitations, a Morse bond :ref:`(Morse) <howto-Morse>` can be
+used in place of the quartic bond, but this improvement when modeling
+bond dissociation is limited when the cross-terms remain harmonic.  To
+improve the physicality of the response of the cross-terms that couple
+bond stretch to other higher-order interactions (angles, dihedrals, and
+impropers), those respective cross-terms also need to be modified.
+
+The cross-term potentials that model bond stretch coupling to
+higher-order interactions are similar in functional form to the harmonic
+bonding potential.  Thus, the exponential function that :ref:`(Morse)
+<howto-Morse>` derived for converting a harmonic bonding potential to a
+bonding potential that accommodates larger bond stretches and can model
+bond dissociation, can also be implemented into the cross-terms.  This
+defines the naming convention for the ClassII-xe functional form, where
+x refers to cross-term and e refers to an exponential function, and
+defines the purpose of the ClassII-xe functional form.  The purpose is
+to allow bond dissociation in a ClassII-based force field via a
+consistent definition of bond dissociation via the Morse bonding
+potential and the higher-order cross-term coupling potentials.  See
+:ref:`(Kemppainen) <howto-Kemppainen>` for a description of the
+ClassII-xe functional form.  The interaction styles listed below compute
+the force field formulas for the ClassII-xe functional form:
+
+* :doc:`bond_style morse <bond_morse>`
+* :doc:`angle_style class2xe <angle_class2>`
+* :doc:`dihedral_style class2xe <dihedral_class2>`
+* :doc:`improper_style class2 <improper_class2>`
+
+* :doc:`pair_style lj/class2 <pair_class2>`
+* :doc:`pair_style lj/class2/coul/cut <pair_class2>`
+* :doc:`pair_style lj/class2/coul/long <pair_class2>`
+
+* :doc:`special_bonds lj/coul 0 0 1 <special_bonds>`
+
+Since both COMPASS and PCFF are Class II-based, their parameters can be
+converted into the ClassII-xe functional form.  The conversion from
+ClassII to ClassII-xe functional form requires reparameterizing the
+cross-terms, which can be accomplished via the LUNAR tool, for which a
+link is provided on the `Pre/Post processing
+<https://www.lammps.org/prepost.html>`_ page.  LUNAR can be used to
+build a model from scratch in either COMPASS or PCFF (using its
+'atom_typing' and 'all2lmp' modules) and then convert that model to
+COMPASS-xe or PCFF-xe (using its 'auto_morse_bond_update' module).  To
+establish a consistent naming convention for the new ClassII-xe
+functional form when converting from a parent force field, the -xe
+suffix should be added to the parent force field name (e.g. PCFF
+vs. PCFF-xe).
+
+The usage of the ClassII-xe functional form can only let a bond
+dissociate; however, to disconnect the dissociated bond and remove the
+higher-order interactions, other LAMMPS commands are required, such as
+:doc:`fix bond/react <fix_bond_react>` or :doc:`fix bond/break
+<fix_bond_break>`.  It was demonstrated that PCFF-xe (without :doc:`fix
+bond/react <fix_bond_react>` or :doc:`fix bond/break <fix_bond_break>`)
+can model a material up to fracture :ref:`(Kemppainen)
+<howto-Kemppainen>`, allowing for calculation of mechanical properties
+such such as tensile and shear modulus, tensile and shear yield
+strength, Poisson's ratio, etc.  However, the simulation may crash after
+fracture if bonds beyond than the processor communication cutoff
+distance.  Thus, post-fracture phenomena cannot be simulated without the
+usage of :doc:`fix bond/react <fix_bond_react>` or :doc:`fix bond/break
+<fix_bond_break>`.  Note that using :doc:`fix bond/break
+<fix_bond_break>` can cause instabilities and crashes because a
+discontinuity is introduced to the energy landscape when a bond is
+removed.  On the other hand, :doc:`fix bond/react <fix_bond_react>` can
+relax high-energy configurations via the ``stabilization`` keyword,
+whereby a small local group of atoms involved in the discontinuity are
+integrated with :doc:`fix nve/limit <fix_nve_limit>`.  `LUNAR
+<https://www.lammps.org/prepost.html>`_ can also be used to assist with
+setting up simulations that include :doc:`fix bond/react
+<fix_bond_react>`.
 
 DREIDING
 --------
@@ -209,28 +297,28 @@ The interaction styles listed below compute force field formulas that
 are consistent with the DREIDING force field.  See each command's
 documentation for the formula it computes.
 
-* :doc:`bond_style <bond_harmonic>` harmonic
-* :doc:`bond_style <bond_morse>` morse
+* :doc:`bond_style harmonic <bond_harmonic>`
+* :doc:`bond_style morse <bond_morse>`
 
-* :doc:`angle_style <angle_cosine_squared>` cosine/squared
-* :doc:`angle_style <angle_harmonic>` harmonic
-* :doc:`angle_style <angle_cosine>` cosine
-* :doc:`angle_style <angle_cosine_periodic>` cosine/periodic
+* :doc:`angle_style cosine/squared <angle_cosine_squared>`
+* :doc:`angle_style harmonic <angle_harmonic>`
+* :doc:`angle_style cosine <angle_cosine>`
+* :doc:`angle_style cosine/periodic <angle_cosine_periodic>`
 
-* :doc:`dihedral_style <dihedral_charmm>` charmm
-* :doc:`improper_style <improper_umbrella>` umbrella
+* :doc:`dihedral_style charmm <dihedral_charmm>`
+* :doc:`improper_style umbrella <improper_umbrella>`
 
-* :doc:`pair_style <pair_buck>` buck
-* :doc:`pair_style <pair_buck>` buck/coul/cut
-* :doc:`pair_style <pair_buck>` buck/coul/long
-* :doc:`pair_style <pair_lj>` lj/cut
-* :doc:`pair_style <pair_lj_cut_coul>` lj/cut/coul/cut
-* :doc:`pair_style <pair_lj_cut_coul>` lj/cut/coul/long
+* :doc:`pair_style buck <pair_buck>`
+* :doc:`pair_style buck/coul/cut <pair_buck>`
+* :doc:`pair_style buck/coul/long <pair_buck>`
+* :doc:`pair_style lj/cut <pair_lj>`
+* :doc:`pair_style lj/cut/coul/cut <pair_lj_cut_coul>`
+* :doc:`pair_style lj/cut/coul/long <pair_lj_cut_coul>`
 
-* :doc:`pair_style <pair_hbond_dreiding>` hbond/dreiding/lj
-* :doc:`pair_style <pair_hbond_dreiding>` hbond/dreiding/morse
+* :doc:`pair_style hbond/dreiding/lj <pair_hbond_dreiding>`
+* :doc:`pair_style hbond/dreiding/morse <pair_hbond_dreiding>`
 
-* :doc:`special_bonds <special_bonds>` dreiding
+* :doc:`special_bonds dreiding <special_bonds>`
 
 OPLS
 ----
@@ -255,16 +343,16 @@ are fully or in part consistent with the OPLS style force fields.  See
 each command's documentation for the formula it computes.  Some are only
 compatible with a subset of OPLS interactions.
 
-* :doc:`bond_style <bond_harmonic>` harmonic
-* :doc:`angle_style <angle_harmonic>` harmonic
-* :doc:`dihedral_style <dihedral_opls>` opls
-* :doc:`improper_style <improper_cvff>` cvff
-* :doc:`improper_style <improper_fourier>` fourier
-* :doc:`improper_style <improper_harmonic>` harmonic
-* :doc:`pair_style <pair_lj_cut_coul>` lj/cut/coul/cut
-* :doc:`pair_style <pair_lj_cut_coul>` lj/cut/coul/long
-* :doc:`pair_modify <pair_modify>` geometric
-* :doc:`special_bonds <special_bonds>` lj/coul 0.0 0.0 0.5
+* :doc:`bond_style harmonic <bond_harmonic>`
+* :doc:`angle_style harmonic <angle_harmonic>`
+* :doc:`dihedral_style opls <dihedral_opls>`
+* :doc:`improper_style cvff <improper_cvff>`
+* :doc:`improper_style fourier <improper_fourier>`
+* :doc:`improper_style harmonic <improper_harmonic>`
+* :doc:`pair_style lj/cut/coul/cut <pair_lj_cut_coul>`
+* :doc:`pair_style lj/cut/coul/long <pair_lj_cut_coul>`
+* :doc:`pair_modify geometric <pair_modify>`
+* :doc:`special_bonds lj/coul 0.0 0.0 0.5 <special_bonds>`
 
 ----------
 
@@ -295,3 +383,15 @@ compatible with a subset of OPLS interactions.
 .. _howto-Jorgensen:
 
 **(Jorgensen)** Jorgensen, Tirado-Rives (1988). J Am Chem Soc, 110, 1657-1666. https://doi.org/10.1021/ja00214a001
+
+.. _howto-Maple:
+
+**(Maple)** Maple, Journal of Computational Chemistry, 15, 162-182 (1994). https://doi.org/10.1002/jcc.540150207
+
+.. _howto-Morse:
+
+**(Morse)** Morse, Physical review, 34, 57 (1929). https://doi.org/10.1103/PhysRev.34.57
+
+.. _howto-Kemppainen:
+
+**(Kemppainen)** Kemppainen, npj Computational Materials 11, 341 (2025). https://doi.org/10.1038/s41524-025-01838-5

doc/src/Howto_viz.rst

@@ -621,7 +621,7 @@ follows the documented steps, those trajectory dump files can be
 `imported and visualized in OVITO
 <https://www.ovito.org/manual/advanced_topics/aspherical_particles.html>`_
 
-.. versionchanged:: TBD
+.. versionchanged:: 30Mar2026
 
    Now uses curved triangles instead of flat ones; "both" option is removed; support for superellipsoids was added
 
@@ -673,7 +673,7 @@ commands to the ``in.ellipse.resquared`` input example:
         color map1 0.459 0.055 0.075 color map2 0.000 0.227 0.427 &
         amap min max cf 0.0 5 min map1 0.1 map1 0.5 white 0.9 map2 max map2
 
-.. versionadded:: TBD
+.. versionadded:: 30Mar2026
 
 The visualization of superellipsoids works exactly the same way as for
 ellipsoids by creating a triangle mesh of an icosahedron and refining