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Alex Granovsky
gran@classic.chem.msu.su

the dipole moment of a charged system is not uniquely defined.
It depends on the choice of a coordinate system.

During optimization/saddle point location/hessian runs
Firefly always computes dipole moment using the origin of
an input coordinate system i.e. at the point with (0,0,0)
coordinates.

To compute dipole moment at the center of mass coordinates,
extract the desired geometry and run runtyp=energy run, adding
the following to your input:

 $ELMOM  IEMOM=1 where=comass $END

Hope this helps.

Kind regards,
Alex Granovsky




On Wed Nov 18 '15 6:24pm, Mihai Medeleanu wrote
-----------------------------------------------
>Hello Firefly users!

>I have the following problem: when I compute the dipole moment for protonated amines (for instance methylammonium cation) I obtain a value ten times greater than with GAMESS-US or other quantum chemistry program. For the mentioned compound Firefly (8.1.1) gave me 15.5157 Debye (which in any case is a too big value!). For non-protonated amines the results are OK (same results on both programs). I used DFT B3LYP 6-31G.
>Data for methylammonium cation:

>!   File created by the GAMESS Input Deck Generator Plugin for Avogadro
> $BASIS GBASIS=N31 NGAUSS=6 NDFUNC=1 NPFUNC=1 $END
> $CONTRL SCFTYP=ROHF RUNTYP=OPTIMIZE DFTTYP=B3LYP MULT=1 ICHARG=1 $END
> $STATPT OPTTOL=0.0001 NSTEP=20 $END

> $DATA
>maH
>C1
>C     6.0    -4.04405     1.60919    -0.01255
>N     7.0    -2.50801     1.70111     0.00782
>H     1.0    -4.43069     0.72487    -0.54256
>H     1.0    -4.44207     2.50496    -0.50704
>H     1.0    -4.43860     1.59341     1.01009
>H     1.0    -2.05318     1.66836    -0.91632
>H     1.0    -2.04559     0.90265     0.50765
>H     1.0    -2.14817     2.57106     0.42741
> $END

>