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Andrey
andrei_ipatov@mail.ru

! ================================================================    
! Input file for FireFly    
! ================================================================    
$CONTRL SCFTYP=MCSCF RUNTYP=ENERGY EXETYP=RUN $END
$SYSTEM MWORDS=12 $END  
$CONTRL COORD=UNIQUE UNITS=ANGS $END 
$MCSCF CISTEP=ALDET SOSCF=1 $END
$DET GROUP=C1 NSTATE=20 NCORE=2 NELS=12 NACT=8 $END
 
$DATA
Oxygen molecule
Dnh  4
 
O  8.000000 0.000000 0.000000 -0.6035
! here was the basis set data !
           
$END

$END
$GUESS GUESS=MOREAD NORB=40 $END
$VEC
! here were the guess vectors !
$END


I made a set of calculations with different basises,
the basis from the internal GAMESS databese 6-311G** (the smallest one among the tested ones), then with the basis sets from the EMSL web page :
PVTZ-Gamess, aug-cc-pVDZ, aug-cc-pVDZ-DK, aug-cc-pVTZ and finally the totally uncontracted aug-cc-pVTZ. I expected that the ground state (triplet X³\Sigma⁻_g ) energy and the energies of the lowest excited states ( singlets a¹\Delta_g and b¹\Sigma⁺_g ) will lower with increase of the basis size, but instead I got the minimal values for the PVTZ-Gamess case and, on the contrary, increase of the total energies for aug-cc-pVDZ and aug-cc-pVDZ-DK.

Moreover, for the larger basis sets, aug-cc-pVTZ, in the converged situation I see, that the double degeneracy of the \Delta state (and also for the other higher states with the angular momentum projection > 0, that must be doubly degenerated in the case of O2 molecule) has disappeared, i.e. these levels arise to be splitted ! In contrast, for the smaller basis sets the degeneracy was practically perfect.

What can be the reason of such strange unphysical dependence ? What I am doing in a wrong way ?

Thanks in advance for any help.
Sincerely
Andrey