Added documentation and make number operator method

src/tequila/quantumchemistry/qc_base.py

@@ -646,16 +646,32 @@ def n_electrons(self) -> int:
         return 2 * len(self.integral_manager.active_reference_orbitals)
 
     def make_annihilation_op(self, orbital, coefficient=1.0):
+        """
+        Compute annihilation operator on orbital=orbital in qubit representation
+        """
         assert orbital<=self.n_orbitals*2
         aop = openfermion.ops.FermionOperator(f'{orbital}', coefficient)
         return self.transformation(aop)
 
     def make_creation_op(self, orbital, coefficient=1.0):
+        """
+        Compute creation operator on orbital=orbital in qubit representation
+        """
         assert orbital<=self.n_orbitals*2
         cop = openfermion.ops.FermionOperator(f'{orbital}^', coefficient)
         return self.transformation(cop)
+
+    def make_number_op(self, orbital):
+        """
+        Compute number operator on orbital=orbital in qubit representation
+        """
+        num_op = self.make_creation_op(orbital) * self.make_annihilation_op(orbital)
+        return num_op
 
     def make_sz_op(self):
+        """
+        Compute the spin_z operator of the molecule in qubit representation
+        """
         sz = QubitHamiltonian()
         for i in range(0, self.n_orbitals * 2, 2):
             one = 0.5 * self.make_creation_op(i) * self.make_annihilation_op(i)
@@ -664,18 +680,27 @@ def make_sz_op(self):
         return sz
 
     def make_sp_op(self):
+        """
+        Compute the spin+ operator of the molecule in qubit representation
+        """
         sp = QubitHamiltonian()
         for i in range(self.n_orbitals):
             sp += self.make_creation_op(i*2) * self.make_annihilation_op(i*2 + 1)
         return sp
 
     def make_sm_op(self):
+        """
+        Compute the spin- operator of the molecule in qubit representation
+        """
         sm = QubitHamiltonian()
         for i in range(self.n_orbitals):
             sm += self.make_creation_op(i*2 + 1) * self.make_annihilation_op(i*2)
         return sm
 
     def make_s2_op(self):
+        """
+        Compute the spin^2 operator of the molecule in qubit representation
+        """
         s2_op = self.make_sm_op() * self.make_sp_op() + self.make_sz_op() * (self.make_sz_op() + 1)
         return s2_op