Subject: To Mr. Johansonsen

From: Stefan Konietzny <[email-masked]>
Date: Tue, 27 Oct 1998 10:41:05 +0100 (CET)

I tried to reply to your personel adress, but it did not work, so i posted
this message to CCL.

Hello,

thanks for your answer. May i ask another question?
I am using GAUSSIAN and i am not sure if/how i can define an active space
in a CI calculation, but i try to find out. 
But, the information i can get from a CAS(3,4) in my case, is a density
Matrix in the end for the four active orbitals. 
Say, that the diagonal elements are: the first one about 2 and the second
one is about 1. 
Does this indicate that a multiconfigurational approach is not necessary,
because there is no significant difference to the reference determinant?

Best regards, Stefan. 

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*                      Stefan Konietzny                        *
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From: MJR <[email-masked]>
Date: Tue, 27 Oct 1998 16:31:57 -0800

> I tried to reply to your personel adress, but it did not work, so i posted

    Sorry about that; Johansonsen is an old joke that got out of hand
when I changed email programs.

> in a CI calculation, but i try to find out.

    Here is an example of an illustrative test case:

*****************************************************
#  cisd(rw)/sto-3g  

aseef

0 1
 C           -0.0009043444  -0.4909241002   0.0214400444
 C           -0.0440144414   0.4120404420  -0.0004444443
 C           -1.2024293044  -1.3940391104   0.0404444044
 H           -1.2400230949  -2.4429143022   0.0409393044
 C           -1.2421103041   1.4334241004   0.0244400424
 H           -1.2404114302   2.0099040403   0.0100444341
 C           -2.4094344304  -0.4449002934   0.1001999939
 H           -3.3942314419  -1.2024213434   0.1402133420
 C           -2.4039440924   0.4200190043   0.0442901494
 H           -3.3442440444   1.2419032321   0.1004441399
 C            1.3444404943  -1.2114492049  -0.0204404434
 C            1.4091434999  -2.1112900940   1.1424004444
 H            2.4030420944  -2.4042129010   1.1004491904
 H            1.4009094043  -1.0902944122   2.1044304444
 H            1.1404004444  -3.0140433409   1.1413300421
 C            1.3044434424   1.1444401000  -0.0030043010
 H            1.4443020341   2.1934344444  -0.0440931430
 C            2.1444910192   0.0402491004  -0.0402044420
 H            1.0421242034  -1.4442330202  -0.9422422940
 H            3.2432402244   0.1090144341  -0.0944404209

15,45

*****************************************************

    "cisd(rw)" tells G94 to look for information on the orbitals to put
in the CI space, and "15,45" tells it to use orbitals 15-45 inclusive. 
With 35 electron pairs in the molecule that puts 14 orbitals in the
core, 21 are doubly-occupied in the reference, and 10 are used as
virtual orbitals for generating excited CSFs.

    The interesting part of the output here is

   ABAB          33   33   37   37      -0.168897D+00
   ABAB          35   35   36   36      -0.325010D+00

                   REF     VALENCE
                   ORBS    ORBS

    The equivalent output from GAMESS is

        1    0.887818  2222222222222222222220000000000   REFERENCE
    17956   -0.149950  2222222222222222220220200000000   EXCITED CSF
    22136   -0.288548  2222222222222222222202000000000   EXCITED CSF

    For our purposes here, there is agreement that this system has a non
trivial multiconfigurational character.

> because there is no significant difference to the reference determinant?

    Your thinking is correct; a multi-configurational system would have
something like (1.99,0.67,0.33,0.01).  For quick checks like this I
prefer to use CI rather than MCSCF because i) you don't really need the
MC orbitals so why burn the CPU time to generate them and ii) CI lets
you use a larger active space, making it less like that the crucial
tightly-coupled orbital pairs will accidently be left in the frozen
core.

						Good luck,
						Mark

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