Friday, December 11, 2009

Common error messages in GAMESS: charge and multiplicity

  *** CHECK YOUR INPUT CHARGE AND MULTIPLICITY ***
THERE ARE 33 ELECTRONS, WITH CHARGE ICHARG= 0
BUT YOU SELECTED MULTIPLICITY MULT= 1
This error message was produced by the following input file. Can you see what's wrong?

$contrl runtyp=optimize $end
$basis gbasis=pm3 $end
$data
Title
C1
N 7.0 -0.39094 1.95659 0.14008
H 1.0 0.38874 1.60529 -0.40413
H 1.0 -0.08386 2.76975 0.70945
H 1.0 -0.72485 1.22934 0.80007
H 1.0 -1.14035 2.30329 -0.48754
O 8.0 -0.64579 0.16732 2.03360
H 1.0 -0.26212 -0.73042 2.10569
H 1.0 -1.00756 0.26750 2.93979
O 8.0 -1.80535 3.31298 -1.59619
H 1.0 -1.39440 3.81065 -2.33214
H 1.0 -2.74148 3.57968 -1.71559
O 8.0 0.26578 4.05264 1.54485
H 1.0 1.03270 4.27032 2.11226
H 1.0 -0.26135 4.87344 1.64760
$end
As GAMESS tells you, it is not possible to have singlet state (i.e. a multiplicity of 1) if you have an odd number of electrons. You need to tell GAMESS whether you want to change the multiplicity (for example, $contrl mult=2 scftyp=rohf $end) or add ($contrl icharg=-1 $end) or remove ($contrl icharg=1 $end).

When looking at the molecule it is pretty clear that you want to remove an electron, since ammonium has a positive charge. So the first line in the input should be changed to
 $contrl runtyp=optimize icharg=1 $end


Click on the picture for an interactive version

Notice that you don't have to specify that the positive charge is on ammonium. The quantum mechanics will take care of that.
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