one zero frequency mode is missing

job314

I am used to seeing 3 zero frequency modes in the crystal frequency calculation output. This urea crystal calculation has only two - I was wondering why and can I use this outcome for thermodynamics calculations with this missing mode

MODES         EIGV          FREQUENCIES     IRREP  IR   INTENS    RAMAN
             (HARTREE**2)   (CM**-1)     (THZ)             (KM/MOL)
    1-   2   -0.1181E-19      0.0000    0.0000  (E  )   A (     0.00)   A
    3-   3    0.2094E-19      0.0000    0.0000  (B2 )   A (     0.00)   A
    4-   4    0.1254E-06     77.7189    2.3300  (B1 )   I (     0.00)   A
    5-   5    0.2460E-06    108.8462    3.2631  (A2 )   I (     0.00)   I
    6-   7    0.3349E-06    127.0111    3.8077  (E  )   A (     0.00)   A
    8-   8    0.4355E-06    144.8406    4.3422  (A1 )   I (     0.00)   A
    9-  10    0.6407E-06    175.6769    5.2667  (E  )   A (     0.00)   A
   11-  12    0.1108E-05    230.9837    6.9247  (E  )   A (     0.00)   A
   13-  13    0.4844E-05    483.0526   14.4816  (B1 )   I (     0.00)   A
   14-  14    0.6693E-05    567.8172   17.0227  (A1 )   I (     0.00)   A
   15-  16    0.6926E-05    577.5886   17.3157  (E  )   A (     0.00)   A
   17-  17    0.7310E-05    593.3990   17.7897  (B2 )   A (     0.00)   A
   18-  19    0.7970E-05    619.6054   18.5753  (E  )   A (     0.00)   A
   20-  20    0.8638E-05    645.0548   19.3383  (A2 )   I (     0.00)   I
   21-  21    0.1067E-04    717.0169   21.4956  (B1 )   I (     0.00)   A
   22-  22    0.1286E-04    787.0371   23.5948  (A2 )   I (     0.00)   I
   23-  24    0.1322E-04    798.0946   23.9263  (E  )   A (     0.00)   A
   25-  26    0.1420E-04    826.9504   24.7914  (E  )   A (     0.00)   A
   27-  27    0.2232E-04   1036.9495   31.0870  (B2 )   A (     0.00)   A
   28-  28    0.2251E-04   1041.3084   31.2176  (A1 )   I (     0.00)   A
   29-  30    0.2535E-04   1105.1276   33.1309  (E  )   A (     0.00)   A
   31-  31    0.2780E-04   1157.1639   34.6909  (B2 )   A (     0.00)   A
   32-  32    0.3003E-04   1202.7304   36.0569  (A1 )   I (     0.00)   A
   33-  34    0.4803E-04   1521.0625   45.6003  (E  )   A (     0.00)   A
   35-  35    0.4892E-04   1535.0190   46.0187  (A1 )   I (     0.00)   A
   36-  36    0.5334E-04   1602.8976   48.0537  (B2 )   A (     0.00)   A
   37-  38    0.5717E-04   1659.4784   49.7499  (E  )   A (     0.00)   A
   39-  39    0.5840E-04   1677.2645   50.2831  (A1 )   I (     0.00)   A
   40-  40    0.6098E-04   1713.8726   51.3806  (B2 )   A (     0.00)   A
   41-  42    0.2380E-03   3386.0936  101.5125  (E  )   A (     0.00)   A
   43-  43    0.2424E-03   3416.7391  102.4313  (A1 )   I (     0.00)   A
   44-  44    0.2438E-03   3426.7219  102.7305  (B2 )   A (     0.00)   A
   45-  46    0.2612E-03   3547.0252  106.3371  (E  )   A (     0.00)   A
   47-  47    0.2614E-03   3548.4246  106.3791  (A1 )   I (     0.00)   A
   48-  48    0.2657E-03   3577.3918  107.2475  (B2 )   A (     0.00)   A

urea.out

dmitoli

Dear Jonas,

What you are seeing in your output is actually consistent with having three zero-frequency acoustic modes. The first entry of the table (modes 1-2) has irrep E, which is a two-dimensional irreducible representation, so it corresponds to two degenerate modes at zero frequency. The second entry (mode 3) has irrep B2, which is non-degenerate and represents the third zero-frequency mode.

This is standard for crystals with symmetry: degeneracies can make the number of printed lines smaller than the actual number of modes.

Hope this helps!

job314

I see, thank you