Handle the case where selections create an empty TensorMap

rascaline/src/calculator.rs

@@ -519,7 +519,9 @@ impl Calculator {
 
         let mut tensor = self.prepare(systems, options)?;
 
-        self.implementation.compute(systems, &mut tensor)?;
+        if tensor.keys().count() > 0 {
+            self.implementation.compute(systems, &mut tensor)?;
+        }
 
         return Ok(tensor);
     }

rascaline/src/calculators/soap/power_spectrum.rs

@@ -533,6 +533,8 @@ impl CalculatorBase for SoapPowerSpectrum {
     #[time_graph::instrument(name = "SoapPowerSpectrum::compute")]
     #[allow(clippy::too_many_lines)]
     fn compute(&mut self, systems: &mut [Box<dyn System>], descriptor: &mut TensorMap) -> Result<(), Error> {
+        assert!(descriptor.keys().count() > 0);
+
         let mut gradients = Vec::new();
         if descriptor.block_by_id(0).gradient("positions").is_some() {
             gradients.push("positions");

rascaline/src/calculators/soap/radial_spectrum.rs

@@ -210,6 +210,8 @@ impl CalculatorBase for SoapRadialSpectrum {
     #[time_graph::instrument(name = "SoapRadialSpectrum::compute")]
     fn compute(&mut self, systems: &mut [Box<dyn System>], descriptor: &mut TensorMap) -> Result<(), Error> {
         assert_eq!(descriptor.keys().names(), ["center_type", "neighbor_type"]);
+        assert!(descriptor.keys().count() > 0);
+
         let mut gradients = Vec::new();
         if descriptor.block_by_id(0).gradient("positions").is_some() {
             gradients.push("positions");

rascaline/src/calculators/soap/spherical_expansion.rs

@@ -764,6 +764,7 @@ impl CalculatorBase for SphericalExpansion {
     #[time_graph::instrument(name = "SphericalExpansion::compute")]
     fn compute(&mut self, systems: &mut [Box<dyn System>], descriptor: &mut TensorMap) -> Result<(), Error> {
         assert_eq!(descriptor.keys().names(), ["o3_lambda", "o3_sigma", "center_type", "neighbor_type"]);
+        assert!(descriptor.keys().count() > 0);
 
         let do_gradients = GradientsOptions {
             positions: descriptor.block_by_id(0).gradient("positions").is_some(),

rascaline/src/calculators/soap/spherical_expansion_pair.rs

@@ -708,6 +708,7 @@ impl CalculatorBase for SphericalExpansionByPair {
     #[time_graph::instrument(name = "SphericalExpansionByPair::compute")]
     fn compute(&mut self, systems: &mut [Box<dyn System>], descriptor: &mut TensorMap) -> Result<(), Error> {
         assert_eq!(descriptor.keys().names(), ["o3_lambda", "o3_sigma", "first_atom_type", "second_atom_type"]);
+        assert!(descriptor.keys().count() > 0);
 
         let do_gradients = GradientsOptions {
             positions: descriptor.block_by_id(0).gradient("positions").is_some(),

rascaline/src/tutorials/moments/moments.rs

@@ -102,6 +102,7 @@ impl CalculatorBase for GeometricMoments {
     // [compute]
     fn compute(&mut self, systems: &mut [Box<dyn System>], descriptor: &mut TensorMap) -> Result<(), Error> {
         assert_eq!(descriptor.keys().names(), ["center_type", "neighbor_type"]);
+        assert!(descriptor.keys().count() > 0);
 
         let do_positions_gradients = descriptor.block_by_id(0).gradient("positions").is_some();
 

rascaline/src/tutorials/moments/s3_compute_5.rs

@@ -69,7 +69,8 @@ impl CalculatorBase for GeometricMoments {
     fn compute(&mut self, systems: &mut [Box<dyn System>], descriptor: &mut TensorMap) -> Result<(), Error> {
         // ...
 
-        // add this line
+        // add these lines
+        assert!(descriptor.keys().count() > 0);
         let do_positions_gradients = descriptor.block_by_id(0).gradient("positions").is_some();
 
         for (system_i, system) in systems.iter_mut().enumerate() {