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Additional XMCQDPT questions

Thomas
thomaspijper@hotmail.com


Dear Alex, dear Firefly-ers,

First of all, many thanks for the post on (X)MCQDPT earlier this month ( http://classic.chem.msu.su/cgi-bin/ceilidh.exe/gran/gamess/forum/?C35e9ea936bHW-7650-1219+00.htm ). It has been of great help to understand this technique and apply it to some smaller molecules. However, for the system Iím currently investigating I would like to ask for some additional information.

For the system under investigation Iím interested in finding the energies for the 3 lowest states. The system has 4 pi bonds and 2 lone pairs which are in conjugation with the pi system, so Iíve opted for a (12,10) active space which includes all p orbitals, with state averaging over the 3 lowest states. I then did a XMCQDPT2 calculation with the following parameters:

$XMCQDPT NSTATE=15 EDSHFT=0.02 IROT=1 THRGEN=1D-10 WSTATE(1)=1,1,1,-0 AVECOE(1)=1,1,1,-0 $END

However, the results are a bit surprising when I look at the eigenvectors of the effective Hamiltonian (see attachment for full input and output files):

       1          2          3
***********************************

 1  0.979176   0.088076  -0.026794
 2  0.035958   0.059822  -0.016755
 3 -0.035267   0.704873   0.694772
 4  0.010869   0.094671   0.009798
 5  0.129268   0.135014  -0.201877
 6  0.033920   0.143020  -0.159014
 7  0.014135   0.021763  -0.020772
 8  0.001207   0.004650  -0.018419
 9 -0.048716   0.049059  -0.021054
10 -0.085162   0.248572  -0.207512
11 -0.056139   0.282944  -0.224556
12  0.054204  -0.289349   0.328600
13 -0.066811   0.447878  -0.487557
14  0.016651  -0.100756   0.093465
15  0.006428  -0.070395  -0.032100 

Apparently, the CASSCF states 10 till 13 contribute significantly to XMCQDPT states 2 and 3. My question is: is it common for high-lying states to be so important? Or does it rather show a problem with the underlying CASSCF states?


Then there is a second question I would like to ask. If I were to proceed by including CASSCF states 10-13 (as well as states 5 and 6 which seem to be of importance as well) in both the SA-CASSCF as well as the XMCQDPT part of the calculation, I would write the following input:

$DET NSTATE=40 ITERMX=400 WSTATE(1)=1,1,1,0,1,1,0,0,0,1,1,1,1 $END

$XMCQDPT NSTATE=20 EDSHFT=0.02 IROT=1 THRGEN=1D-10 WSTATE(1)=1,1,1,0,1,1,0,0,0,1,1,1,1,-0 AVECOE(1)=1,1,1,0,1,1,0,0,0,1,1,1,1,-0 $END 

However, this gives me problems during the CI optimization: after many iterations, one or more states become unconverged and will not converge again:

     5    -1372.2196226568     0.00000114
     5    -1372.2077781435     0.00017627
     5    -1372.2072391615     0.00525372
     5    -1372.2039774568     0.00568757

    Warning - some CI states may have just been missed!

  -1 CONVERGED CI STATE(S) HAVE BECOME UNCONVERGED, NOW WORKING ON   3 STATES.


After a few more iterations:


    73    -3820.2279388183 30217.75359972
    73    -3677.7641825114 28409.89326395
    73    -2930.8951122879 15383.10968857
    73    -2905.6192085034 14967.16114983
    73    -2821.9694978003 14105.86660842
    73    -1372.4833177907     0.00000041
    73    -1372.4258925356     0.00000015
    73    -1372.3874360397     0.00000021
    73    -1372.3748222301     0.00000020
    73    -1372.3321312588     0.00000017
    73    -1372.2919746805     0.00000020
    73    -1372.2521978693     0.00000018 



I encounter this problem quite often for large values of NSTATE (see my other attached input and output files for an example). My question is: how does one solve this problem? I tried increasing the accuracy of the calculation (ICUT=10, INTTYP=HONDO, tighter convergence criteria), but this didnít help.


Many thanks in advance for your help.


Kind regards,
Thom
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