quantum-espresso/examples/example32

This example shows how to use the cpvib.x to calculate the normal modes,
Born charge tensors and infrared cross-sction, of a water molecule in vacuum.

Note that h2o.vib.inp is not read from the standard input and thus has to be 
in "outdir" for the program to find it. Similarly for the isotope file
h2o.vib.isotope.

In this example you should "sample" the two common warnings that the code issues:

1. 
     from calc_hessian : info #        -1
     Warning: Reference structure is not converged!!!

   Issued when the total energy of a perturbed configuration is lower than the
   reference energy of the ground-state configuration. In some cases it is an 
   indication of insufficient structural relaxation, and further relaxation
   is required. However, for very soft (low frequency) modes it is very difficult
   to converge the structure properly. The soft modes include also the 'zero'
   frequency modes associated with rigid translation and rotation. In practice
   these are never zero, for numerical reasons, but could be projected out
   of the final results.

2.
     from calc_hessian : info #        -1
     Warning: consistency check
    Numerical second derivative of the total energy, compared to
    first derivative of the forces, for diagonal hessian element,
    deviate by more then 10%:
        Energy second derivative :    -0.0003
        Force  first  derivative :     0.0004

   The matrix elements of the energy hessian are evaluated by a simple two-point
   formula for the first derivative of the forces. For diagonal matrix elements
   we can compare with the (less accurate) second numerical derivative of the
   total energy, as a consistency check. If the difference between the two estimates 
   is larger than 10% (arbitrarily chosen) the code will issue the above warning, 
   but only if the matrix element is estimated to be larger than 1.0E-4. 
   Very small matrix elements are again associated with soft modes and are hard to
   compute accurately.

In some cases the 'problems' causing the warnings are harmless (like in the present 
example). However one should always verify it in each case.