Add "How to compute the free energy of solids."

sphinx-doc/how-to.rst

@@ -10,8 +10,9 @@ How-to
     howto/determine-the-most-efficient-device
     howto/choose-the-neighbor-list-buffer-distance
     howto/molecular
-    howto/custom-md-potential
     howto/continuously-vary-potential-parameters
+    howto/custom-md-potential
     howto/minimize-potential-energy
     howto/prevent-particles-from-moving
+    howto/compute-the-free-energy-of-solids
     howto/cpppotential

sphinx-doc/howto/compute-the-free-energy-of-solids.rst

@@ -0,0 +1,58 @@
+.. Copyright (c) 2009-2023 The Regents of the University of Michigan.
+.. Part of HOOMD-blue, released under the BSD 3-Clause License.
+
+How to compute the free energy of solids
+========================================
+
+Follow these steps to perform `Frenkel-Ladd`_ (*Einstein crystal*) or `Vega-Noya`_ (*Einstein
+molecule*) approach to compute the free energy of solid phases.
+
+MD simulations
+--------------
+
+1. Initialize your system in the ideal crystal structure.
+2. Add stationary reference particles at the equilibrium sites.
+3. Set the pair force ``r_cut`` to 0 between the reference particle type and all particle types.
+4. Add bonds (`hoomd.md.bond.Harmonic`) between the stationary reference particles and the mobile
+   particles.
+5. Integrate the equations of motion of the mobile particles.
+
+   .. seealso::
+
+       `prevent-particles-from-moving`.
+
+6. Adjust the strength of bonds as needed during the simulation.
+
+   .. seealso::
+
+       `continuously-vary-potential-parameters`.
+
+7. When using the *Einstein crystal* approach, apply `hoomd.update.RemoveDrift` to adjust the center
+   of mass of the system.
+
+HPMC simulations
+----------------
+
+1. Initialize your system in the ideal crystal structure.
+2. Apply the harmonic external potential (`hoomd.hpmc.external.field.Harmonic`), using variants
+   to adjust the spring constants as needed during the simulation.
+3. Apply trial moves to the mobile particles.
+
+   .. seealso::
+
+       `prevent-particles-from-moving`.
+
+4. When using the *Einstein crystal* approach, apply `hoomd.update.RemoveDrift` to adjust the center
+   of mass of the system.
+
+Log and analyze energies
+------------------------
+
+In both MD and HPMC simulations:
+
+1. Log relevant energies (e.g. with `hoomd.write.HDF5Log`).
+2. Post process the logged data to compute the free energy following the procedures in
+   `Frenkel-Ladd`_ or `Vega-Noya`_ as appropriate.
+
+.. _Frenkel-Ladd: https://doi.org/10.1063/1.448024
+.. _Vega-Noya: https://doi.org/10.1063/1.2790426