add some comments

meanas/fdfd/bloch.py

@@ -559,7 +559,8 @@ def eigsolve(
         E_signed = real(trace(ZtAZU))
         sgn = numpy.sign(E_signed)
         E = numpy.abs(E_signed)
-        G = (AZ @ U - Z @ U @ ZtAZU) * sgn
+        G = (AZ @ U - Z @ U @ ZtAZU) * sgn     # G = AZU projected onto the space orthonormal to Z
+                                               #  via (1 - ZUZt)
 
         if i > 0 and abs(E - prev_E) < tolerance * 0.5 * (E + prev_E + 1e-7):
             logger.info('Optimization succeded: '
@@ -568,8 +569,8 @@ def eigsolve(
                 )
             break
 
-        KG = scipy_iop @ G
-        traceGtKG = _rtrace_AtB(G, KG)
+        KG = scipy_iop @ G          # Preconditioned steepest descent direction
+        traceGtKG = _rtrace_AtB(G, KG)      #
 
         if prev_traceGtKG == 0 or i % reset_iters == 0:
             logger.info('CG reset')