[molpro-user] Level shift corrected energies

[A.Zech at stud.uni-heidelberg.de]

Hello all,

whenever I use augmented basis sets in my calculations (CASPT2) i'll  
get an error like "Small denominator detected. Use level shift.". I  
tried different level shift values to check if the energies converge  
to a certain value (I added the data points below if you want to plot  
them). The absolute energies of first (2A1) and second (1B2) excited  
state are increasing and both states eventually cross for a level  
shift of 0.27.
I'm aware that the effect of the level shift on the energy cannot be  
fully removed, but in this case the 2A1 energies for a level shift of  
0.15 and 0.30 respectively, vary significantly (0.4 eV). This problem  
also appears for other CAS spaces. Since the energy gap between the  
excited states is strongly dependent on the level shift i'm not sure  
which level shift value represents the best solution.
Has someone else experienced the same problems with augmented basis  
while using a level shift?

Best regards,

Alexander Zech

------energies as a function of level shift----
shift    1A1                 2A1                 1B2
0.15	-232.67202386751    -232.50099188778	-232.49039691602
0.16	-232.67165404359    -232.49658920931	-232.48975315346
0.17	-232.67126652228    -232.49394382323	-232.48909333701
0.18	-232.67086188366    -232.49200278013	-232.48841815103
0.19	-232.67044068602    -232.49042422423	-232.48772822556
0.20	-232.67000346707    -232.48905931379	-232.48702414595
0.21	-232.66955074481    -232.48782867471	-232.48630646021
0.22	-232.66908301800    -232.48668809747	-232.48557568456
0.23	-232.66860076889    -232.48561037405	-232.48483230773
0.24	-232.66810446214    -232.48457622634	-232.48407679429
0.25	-232.66759454468    -232.48357427921	-232.48330958749
0.26	-232.66707144661    -232.48259465833	-232.48253111111
0.27	-232.66653559247    -232.48163236798	-232.48174177141
0.28	-232.66598738263    -232.48068206580	-232.48094180472
0.29	-232.66542720726    -232.47974029980	-232.48013189811
0.30	-232.66485544361    -232.47880445375	-232.47931239667

------INPUT-----(using Molpro2009)----------------
***,cis-Hexatrien CAS(4,4), shift=0.15

  gprint,basis
  gprint,orbital

  basis=aug-cc-pVDZ,h=cc-pVDZ
  angstrom
  geometry={
  C, 0.0000000000, 0.6846872752, 0.1155367732
  H, 0.0000000000, 1.1963059212,-0.8561850879
  C, 0.0000000000, 1.5464655270, 1.2944328313
  H, 0.0000000000, 1.0808049055, 2.2866004300
  C, 0.0000000000, 2.9032715280, 1.2137053055
  H, 0.0000000000, 3.5262177875, 2.1121985938
  H, 0.0000000000, 3.4098430336, 0.2422991441
  C, 0.0000000000,-0.6846872752, 0.1155367732
  H, 0.0000000000,-1.1963059212,-0.8561850879
  C, 0.0000000000,-1.5464655270, 1.2944328313
  H, 0.0000000000,-1.0808049055, 2.2866004300
  C, 0.0000000000,-2.9032715280, 1.2137053055
  H, 0.0000000000,-3.5262177875, 2.1121985938
  H, 0.0000000000,-3.4098430336, 0.2422991441
  }

  {hf;wf,44,1,0
  orbprint,50}
  {casscf
  closed,10,1,9,0
  occ,10,3,9,2
  save,ref=4000.2
  wf,44,1,0;state,2;wf,44,3,0}
  {rs2c,shift=0.15,maxit=80
  state,2,1,2
  wf,44,1,0
  select,ref1=4000.2,,0.0
  thresh,printci=0.03
  }
  {rs2c,shift=0.15,maxit=80
  state,1,1
  wf,44,3,0
  select,ref1=4100.2,,0.0
  thresh,printci=0.03
  }
  ---