[molpro-user] What is spin ordering in doubly occupied CI vector entries?

Dr. Seth Olsen s.olsen1 at uq.edu.au
Thu Oct 11 00:24:44 BST 2007

Hi Molpro-Users,

I have questions about what conventions Molpro may follow when printing 
out the CI vector from an MCSCF calculation.
1. What is the spin ordering in a doubly occupied orbital?  For example 
if a determinant is listed as '2ab', then are the spins in the doubly 
occupied orbital ordered in the same way as the singly occupied ones - 
'ab' in the above case?  Is there a convention for spin-ordering in 
determinants like '220', where there are not singly occupied states to 
judge by?  It seems this could make a big difference in wavefunction 
analyses, because swapping the spin ordering of a pair of electrons will 
change the sign.  Is there a way to get Molpro to print out a more 
detailed notation for the determinants?

2. I have noticed that almost all of the time, 2 determinants with the 
same number occupations ( '2ab' and '2ba', for example) will print out 
with the 'beta-first' configuration preceeding the 'alpha-first' (that 
is, if '2ab' and '2ba' form a CSF with equal and opposite coefficients, 
then '2ba' will print first, followed by '2ab' with the sign of the 
coefficient reversed).  Is there any significance to rare cases where 
the order is reversed (i.e. where '2ab' prints before '2ba')?  Is this 
just due to numerical inaccuracies which cause the determinants to have 
very small differences in magnitude and which lead the program to print 
the 'higher' magnitude contribution first?

I would really appreciate it if someone would enlighten me on this 
topic.  I've been pouring over a set of CI vectors for weeks to try and 
extract a VB-type analysis of the wavefunction for a multi-state 
calculation.  There are conical interesections involved, and I want to 
be able to figure out what the right sign is for various contributions 
to the wavefunction for each state, and to be able to untangle orbital 
phase changes from CI vector phase changes.  Thank you very much for 
your time.

Many Thanks,



Dr Seth Olsen, PhD
Postdoctoral Fellow, Biomolecular Modeling Group
Centre for Computational Molecular Science
Australian Institute  For Bioengineering and  Nanotechnology  (75)
The University of Queensland
Brisbane Qld 4072 Australia 

Tel: +61 7 3346 3946
Fax: +61 7 3346  3992
email: s.olsen1 at uq.edu.au
Web: www.ccms.uq.edu.au 


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