Dear developer:

I have a question about using the discover module.

when I obtianed the hessian matrix of a crystal（for the period system）, The vib.hessian file is like this:

********************************************************************************************************

@Units

distance: A

energy: Kcal/Mol

#header

@natom 3

#label

H1      O1      H2     

#element

H  O  H 

#matrix

15

2.271151623510e+003

-2.622342517559e+003 6.095245725507e+003

2.837005074105e-004 -4.243273300625e-004 1.100816773187e+002

-2.348215402914e+003 2.250907425715e+003 -2.864458923914e-004 4.696430551315e+003

....................................

#frequencies and intensities

9

......

1936.135034        211.446990

3629.433764        198.962824

3629.960117        154.792053

#normal_modes

9

  0.0000000077 -0.0000000363 -0.7071067988 -0.0000000000  0.0000000181  0.0000000089 -0.0000000077 -0.0000000360  0.7071067635

  ..........................

  0.0003565970 -0.0065837964  0.2364438591  0.0014207319 -0.0262307617  0.9420252569  0.0003565970 -0.0065837964  0.2364438591

**********************************************************************

it is easy to see that there are 3 atoms in this crystal, therefore the total number of eiegenvalues should be 9 and the vibrational mode is 3N-6=3,where N is the number of atoms.

the question is that the dimension of hessian matrix is 15 in this vib.hessian file not 9. Would you please explain why the dimension of this hessian is bigger that 3N.

I am willing to know the details in the algorithm about the reduction from 15D to 9D in hessian matrix, and then get the nine correct frequencies as mentioned above
by diagonaling the smaller 9d hessian matrix.

Looking forward your reply.

Yours sincerely,

ww