I am running some CASTEP free energy calculations and would like some support on assessing the output.

From my calculations on a small molecule I get a negative phonon frequency, which normally means the structure is not on a minimum or something is wrong with the calculations…

==============================================================================

+                           Vibrational Frequencies                          +

+                           -----------------------                          +

+                                                                            +

+ Performing frequency calculation at  1 wavevector  (q-pt )                 +

+ ========================================================================== +

+                                                                            +

+ -------------------------------------------------------------------------- +

+  q-pt=    1 (  0.000000  0.000000  0.000000)     1.0000000000              +

+ -------------------------------------------------------------------------- +

+  Acoustic sum rule correction <   9.874013 cm-1 applied                    +

+     N      Frequency irrep.    ir intensity active            raman active + 

 +                (cm-1)         ((D/A)**2/amu)                               + 

 +                                                                            +

+     1      -8.683585   a          0.0000000  N                       Y     + 

 +     2      -0.058355   b          0.0000000  N                       N     + 

 +     3      -0.047438   c          0.0000000  N                       N     + 

 +     4      -0.033043   d          0.0026330  N                       N     + 

Can someone explain how the sum rule is applied (eg to all modes or just the 3 translations?) and suggest solutions to having a negative frequency?