I'm using GAMESS to run a geometry optimization on a thiourea molecule, using PM6. However, the output file keeps displaying the error message, "NO FORCE FIELD, SCF DOES NOT CONVERGE." Is there an error in my input code?

    NOSYM=1 $END
C          6  -0.000001   -0.216235   -0.000006
N          7  -1.149850   -0.964256   -0.062205
N          7   1.149844   -0.964254    0.062165
S         16   0.000003    1.474558   -0.000003
C          6  -2.468377   -0.327323    0.016251
C          6   2.468378   -0.327328   -0.016221
H          1  -3.255938   -1.092284   -0.077195
H          1  -2.599975    0.405207   -0.806499
H          1  -2.609097    0.216356    0.970441
H          1   3.255932   -1.092302    0.077168
H          1   2.599974    0.405143    0.806581
H          1   2.609108    0.216419   -0.970372
H          1  -1.133325   -1.963288    0.067490
H          1   1.133319   -1.963297   -0.067428
  • $\begingroup$ In these cases, the first thing to do might be to increase the number of SCF cycles, so try MAXIT=150, and see what happens. $\endgroup$
    – Pier
    Jul 12, 2019 at 0:22
  • $\begingroup$ I am unfamiliar with GAMESS, but make sure you have specified the method correctly. Also, check your input geometry visually and consider switching to another method. Maybe the C-S bond is badly described in PM6. $\endgroup$
    – TAR86
    Jul 12, 2019 at 5:48
  • 1
    $\begingroup$ I couldn't find any indication that pm6 is actually available in GAMESS. Maybe try pm3, and see whether it works. I think pm6 would do fine for the molecule, pm3 probably too. If you require pm6 you might have to try a different ESS code. $\endgroup$ Jul 12, 2019 at 11:28
  • 1
    $\begingroup$ I spent some time yesterday trying it on WebMo (I have no local GAMESS installations on the OS I am working on right now), and even if I am not familiar with GAMESS, it looks like for some weird reason PM6 doesn't like that molecule (increased optimization and scf cycles, modified convergence criteria). If I recall correctly, anyway, it worked with PM3. $\endgroup$
    – user32223
    Jul 13, 2019 at 20:31
  • 1
    $\begingroup$ I believe the problem has to lie within GAMESS. I just ran an optimisation with Gaussian 16 B.01 and pm6, which looks like you'd would expect it. ( cc @The_Vinz ) Could you please specify the version you are using. Have you tried other molecules; did they work? Maybe good choices to localise the problem would be urea, CS2/CS, maybe the problem lies there. I'll see if I can get GAMESS to work, then I might try again. $\endgroup$ Jul 23, 2019 at 11:47

1 Answer 1


TL;DR: PM6 only works for elements up to fluoride in GAMESS.

I have played around with the GAMESS version installed at our cluster, the ancient 5 DEC 2014 (R1) and I came to the conclusion that there must be a 'bug' in the implementation of PM6 with GAMESS. (I reproduced the problem with 20 APR 2017 (R1); I have also downloaded 30 Jun 2019 (R1), but I won't try it as I think the result is the same.) When I tried it with Gaussian 16 Rev. B.01 on the PM6 level of theory it converged fine.

Actually, I have looked at the documentation, and it seems to be officially not supported (from INTRO.DOC):


   3. Calculates semi-empirical MNDO, AM1, or PM3 models 
      using RHF, UHF, ROHF, or GVB wavefunctions.

Or from INPUT.DOC:

        * * * semiempirical basis sets * * *

GBASIS = MNDO - selects MNDO model Hamiltonian
       = AM1  - selects AM1 model Hamiltonian
       = PM3  - selects PM3 model Hamiltonian
       = RM1  - selects RM1 model Hamiltonian
       = DFTB - selects tight binding Hamiltonian

I am pretty sure the error is associated with sulfur. I have tried a few molecules, among them $\ce{H3C-NH2}$, $\ce{(H3C)2NH}$, $\ce{(H3C)2CO}$ (see input below), urea, which converged fine. Whenever I tried a molecule containing sulfur the SCF seemed to be oscillating. I have tested $\ce{CS2}$, $\ce{SO2}$, $\ce{(H3C)2CS}$, and (obviously) thiourea. (With PM3 everything worked fine.)

My best guess is that they simply didn't finish implementing it. You could issue a support ticket on GitHub: issues. I found an article by (the here known) Jan Jensen from 2013: Project status: PM6 and PCM in GAMESS. And then from 2014: PM6, DFTB, GAMESS, enzyme design, protein-ligand binding, and protein structure validation. I stopped there.

I'd recommend using a different program, the obvious choice would be MOPAC (http://openmopac.net/). Gaussian has it implemented and a couple others probably too. Or you switch to a completely different method like xtb, HF-3c (in ORCA), DFTB (e.g. DFTB+), etc.. Depending on your system size, you can probably also get DFT based calculations done.

Here is also something related: GAMESS crash course?

    NOSYM=1 $END
CARBON      6.0      0.156332162        -0.059231571         0.417752882
OXYGEN      8.0      1.200569512        -0.535604279         0.831319560
CARBON      6.0     -0.037914597         1.466530485         0.340822109
HYDROGEN    1.0      0.279389785         1.860652999        -0.631337255
HYDROGEN    1.0     -1.089141584         1.742544046         0.481697509
HYDROGEN    1.0      0.545018653         1.982387227         1.112307247
CARBON      6.0     -1.053925162        -0.882111362        -0.061565447
HYDROGEN    1.0     -0.987243509        -1.102851552        -1.132983096
HYDROGEN    1.0     -1.121341433        -1.838731138         0.468944868
HYDROGEN    1.0     -1.993938204        -0.344384630         0.106761952

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