PwBandsWorkChain: make nbands_factor optional (#474)

By making this a non required input, the caller has to opportunity to
explicitly specify a number of bands to be used for the bands step
through the `bands.pw.parameters` input node. One can now specify the
number of bands to be used in the bands step either directly through
the input parameters `bands.pw.parameters.SYSTEM.nbnd` or through
the `nbands_factor` input. Note that specifying both is not allowed.
When neither is specified nothing will be set by the work chain and
the default of Quantum ESPRESSO will end up being used. If the
`nbands_factor` is specified the maximum value of the following values
will be used:

  * `nbnd` of the preceding SCF calculation
  * 0.5 * nspin * nelectrons * nbands_factor
  * 0.5 * nspin * nelectrons + 4 * nspin

aiida_quantumespresso/workflows/pw/bands.py

@@ -16,8 +16,28 @@
 PwRelaxWorkChain = WorkflowFactory('quantumespresso.pw.relax')
 
 
+def validate_inputs(inputs, ctx=None):  # pylint: disable=unused-argument
+    """Validate the inputs of the entire input namespace."""
+    if 'nbands_factor' in inputs and 'nbnd' in inputs['bands']['pw']['parameters'].get_attribute('SYSTEM', {}):
+        return PwBandsWorkChain.exit_codes.ERROR_INVALID_INPUT_NUMBER_OF_BANDS.message  # pylint: disable=no-member
+
+
 class PwBandsWorkChain(WorkChain):
-    """Workchain to compute a band structure for a given structure using Quantum ESPRESSO pw.x."""
+    """Workchain to compute a band structure for a given structure using Quantum ESPRESSO pw.x.
+
+    The logic for the computation of various parameters for the BANDS step is as follows:
+
+    Number of bands:
+        One can specify the number of bands to be used in the BANDS step either directly through the input parameters
+        `bands.pw.parameters.SYSTEM.nbnd` or through `nbands_factor`. Note that specifying both is not allowed. When
+        neither is specified nothing will be set by the work chain and the default of Quantum ESPRESSO will end up being
+        used. If the `nbands_factor` is specified the maximum value of the following values will be used:
+
+        * `nbnd` of the preceding SCF calculation
+        * 0.5 * nspin * nelectrons * nbands_factor
+        * 0.5 * nspin * nelectrons + 4 * nspin
+
+    """
 
     @classmethod
     def define(cls, spec):
@@ -34,8 +54,9 @@ def define(cls, spec):
         spec.input('structure', valid_type=orm.StructureData, help='The inputs structure.')
         spec.input('clean_workdir', valid_type=orm.Bool, default=lambda: orm.Bool(False),
             help='If `True`, work directories of all called calculation will be cleaned at the end of execution.')
-        spec.input('nbands_factor', valid_type=orm.Float, default=lambda: orm.Float(1.2),
+        spec.input('nbands_factor', valid_type=orm.Float, required=False,
             help='The number of bands for the BANDS calculation is that used for the SCF multiplied by this factor.')
+        spec.inputs.validator = validate_inputs
         spec.outline(
             cls.setup,
             if_(cls.should_do_relax)(
@@ -49,12 +70,14 @@ def define(cls, spec):
             cls.inspect_bands,
             cls.results,
         )
+        spec.exit_code(201, 'ERROR_INVALID_INPUT_NUMBER_OF_BANDS',
+            message='Cannot specify both `nbands_factor` and `bands.pw.parameters.SYSTEM.nbnd`.')
         spec.exit_code(401, 'ERROR_SUB_PROCESS_FAILED_RELAX',
-            message='the PwRelaxWorkChain sub process failed')
+            message='The PwRelaxWorkChain sub process failed')
         spec.exit_code(402, 'ERROR_SUB_PROCESS_FAILED_SCF',
-            message='the scf PwBasexWorkChain sub process failed')
+            message='The scf PwBasexWorkChain sub process failed')
         spec.exit_code(403, 'ERROR_SUB_PROCESS_FAILED_BANDS',
-            message='the bands PwBasexWorkChain sub process failed')
+            message='The bands PwBasexWorkChain sub process failed')
         spec.output('primitive_structure', valid_type=orm.StructureData,
             help='The normalized and primitivized structure for which the bands are computed.')
         spec.output('seekpath_parameters', valid_type=orm.Dict,
@@ -141,24 +164,25 @@ def inspect_scf(self):
 
     def run_bands(self):
         """Run the PwBaseWorkChain in bands mode along the path of high-symmetry determined by seekpath."""
-
-        # Get info from SCF on number of electrons and number of spin components
-        scf_out_dict = self.ctx.workchain_scf.outputs.output_parameters.get_dict()
-        nelectron = int(scf_out_dict['number_of_electrons'])
-        nspin = int(scf_out_dict['number_of_spin_components'])
-        nbands = max(
-            int(0.5 * nelectron * nspin * self.inputs.nbands_factor.value),
-            int(0.5 * nelectron * nspin) + 4 * nspin)
-
         inputs = AttributeDict(self.exposed_inputs(PwBaseWorkChain, namespace='bands'))
         inputs.pw.parameters = inputs.pw.parameters.get_dict()
 
         inputs.pw.parameters.setdefault('CONTROL', {})
         inputs.pw.parameters.setdefault('SYSTEM', {})
         inputs.pw.parameters.setdefault('ELECTRONS', {})
 
+        # The following flags always have to be set in the parameters, regardless of what caller specified in the inputs
         inputs.pw.parameters['CONTROL']['calculation'] = 'bands'
-        inputs.pw.parameters['SYSTEM']['nbnd'] = nbands
+
+        # If `nbands_factor` is defined in the inputs we set the `nbnd` parameter
+        if 'nbands_factor' in self.inputs:
+            factor = self.inputs.nbands_factor.value
+            parameters = self.ctx.workchain_scf.outputs.output_parameters.get_dict()
+            nspin = int(parameters['number_of_spin_components'])
+            nbands = int(parameters['number_of_bands'])
+            nelectron = int(parameters['number_of_electrons'])
+            nbnd = max(int(0.5 * nelectron * nspin * factor), int(0.5 * nelectron * nspin) + 4 * nspin, nbands)
+            inputs.pw.parameters['SYSTEM']['nbnd'] = nbnd
 
         # Only set the following parameters if not directly explicitly defined in the inputs
         inputs.pw.parameters['ELECTRONS'].setdefault('diagonalization', 'cg')