📚 Docs: Add first version of PwRelaxWorkChain how to

docs/source/howto/workflows/pw/relax.md

@@ -2,4 +2,101 @@
 
 # `PwRelaxWorkChain`
 
-*To be added.*
+As with all work chains, the best way to get started with running the `PwRelaxWorkChain` is *via* the `get_builder_from_protocol()` method.
+
+First, load the AiiDA orm and activate your profile:
+
+```python
+from aiida import orm, load_profile
+
+load_profile()
+```
+
+Next, load the `PwRelaxWorkChain` and the structure/code you want to run:
+
+```python
+from aiida_quantumespresso.workflows.pw.relax import PwRelaxWorkChain
+
+code = orm.load_code('qe-7.2-pw@localhost')
+structure = orm.load_node(1)  # FCC Silicon
+```
+
+Now you can obtain a fully populated builder using the `get_builder_from_protocol()` method:
+
+```python
+builder = PwRelaxWorkChain.get_builder_from_protocol(
+    code=code,
+    structure=structure
+)
+```
+
+You can check its contents using:
+
+```python
+builder
+```
+
+:::{important}
+The `get_builder_from_protocol()` method provides you with a fully populated builder with sensible defaults for most structures.
+However, it does *not* guarantee convergence for all structures, so it is important to double-check e.g. k-points meshes and plane wave cutoffs for important results.
+:::
+
+Now that you have a fully-populated builder that is ready to run, load the `submit` function from the AiiDA engine and launch the process!
+
+```python
+from aiida.engine import submit
+
+submit(builder)
+```
+
+## Defining the `RelaxType`
+
+By default, the `get_builder_from_protocol()` method will set inputs that instruct Quantum ESPRESSO to optimise both the geometry and cell, i.e.:
+
+* `CONTROL.calculation = vc-relax`
+* `CELL.conv_thr = all`
+
+You can of course change these inputs by adapting the `builder` afterwards, for example:
+
+```python
+builder = PwRelaxWorkChain.get_builder_from_protocol(
+    code=code,
+    structure=structure
+)
+
+parameters = builder.base.pw.parameters.get_dict()
+parameters['CONTROL']['calculation'] = 'relax'
+parameters.pop('CELL')
+builder.base.pw.parameters = orm.Dict(parameters)
+```
+
+```python
+from aiida.engine import submit
+
+submit(builder)
+```
+
+However, it is somewhat tedious to have to adapt several parameters, and this requires knowledge about the correct combinations of inputs in Quantum ESPRESSO.
+To make it easier to specify what degrees of freedom to allow during the geometry optimization, you can also use the `RelaxType` enum:
+
+```python
+from aiida_quantumespresso.common.types import RelaxType
+
+builder = PwRelaxWorkChain.get_builder_from_protocol(
+    code=code,
+    structure=structure,
+    relax_type=RelaxType.POSITIONS
+)
+```
+
+Check the contents of the `builder`!
+You'll see that the same inputs as above are now set, so only the _positions_ of the atoms are optimized.
+Here is the list of settings for `RelaxType` that are currently supported:
+
+* `POSITIONS`; Only the atomic positions are relaxed, cell is fixed.
+* `SHAPE`: Only the cell shape is optimized at a fixed volume and fixed atomic positions.
+* `CELL`: Only the cell is optimized, both shape and volume, while atomic positions are fixed.
+* `POSITIONS_SHAPE`: Same as `SHAPE`  but atomic positions are relaxed as well.
+* `POSITIONS_CELL`: Same as `CELL`  but atomic positions are relaxed as well.
+
+As was noted above, the default setting here is to optimized both the positions and the unit cell, i.e. `RelaxType.POSITIONS_CELL`.