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Slawomir Janicki
slawomir.janicki@comcast.net

I was comparing analytical and numeric methods for hessian runs, and I found that in one case the numeric methods produced negative frequencies:

geometry optimization run:
$STATPT NSTEP=500 HSSEND=.F. TRMIN=0.01 METHOD=GDIIS NOREG=5
OPTTOL=0.0000001 $END

hessian runs:
analytical:
$FORCE METHOD=ANALYTIC PROJCT=.TRUE. VIBANL=.TRUE. PRTSCN=.TRUE. SCLFAC=1 $END
gave:
                         1           2           3           4           5
      FREQUENCY:         0.00        0.00        0.00        0.00        0.00  

and

         FREQUENCY   ENTROPY   %-CONTRIBUTION
         ---------   -------   --------------
           237.43     1.822           26.50
           237.43     1.822           26.50
           268.28     1.607           23.38
           544.38     0.556            8.09


numeric 1 step:
$FORCE PROJCT=.TRUE. VIBANL=.TRUE. PRTSCN=.TRUE. SCLFAC=1 METHOD=NUMERIC
NVIB=1 $END
gave:
                         1           2           3           4           5
      FREQUENCY:        14.32 I      0.00        0.00        0.00        0.00  

and

         FREQUENCY   ENTROPY   %-CONTRIBUTION
         ---------   -------   --------------
           207.78     2.063           29.17
           212.70     2.020           28.56
           355.72     1.141           16.13
           355.99     1.140           16.11

numeric 2 step:
$FORCE PROJCT=.TRUE. VIBANL=.TRUE. PRTSCN=.TRUE. SCLFAC=1 NVIB=2
METHOD=NUMERIC $END
gave:
                         1           2           3           4           5
      FREQUENCY:        14.14 I      0.00        0.00        0.00        0.00  

and

         FREQUENCY   ENTROPY   %-CONTRIBUTION
         ---------   -------   --------------
           202.41     2.111           29.15
           202.51     2.110           29.13
           352.25     1.156           15.96
           352.31     1.156           15.96

Is there a way to avoid this? I need to rely on numeric requencies when I can't use analytical hessian.

Slawomir