[molpro-user] diabatisation documentation

[Christian Evenhuis]

Dear all,

I am trying to find out exactly how the DDR program works in molpro.   
In short, my questions are:
1. What is the reference J. Chem. Phys. 102, 0000, (1999)  in section  
20.5.8.
2. How does DDR calculate the unitary transformation matrix.
3. What is the first orbital correction correction to the orbitals  
mentioned in section 39?  How is this calculated?

For more context see below.

Thanks,

Chris Evenhuis


Firstly, the CASSCF orbitals are rotated to maximise the overlap with  
the reference orbitals using the diabatisation from section 20.5.8.   
 From section 20.5.8:

"Recently, an automatic procedure, as described in J. Chem. Phys.  
102, 0000, (1999) has been implemented into MOLPRO. This is available  
in Version 99.1 and later and is described in section 35."

In section 35 there is a reference to D. Simah, B. Hartke, and H.-J.  
Werner, J. Chem. Phys. 111, 4523 (1999). Is this the mystery  
reference from above?  From the description I see that the CASSCF  
orbitals are diabatised by either 2x2 Jacobi rotations or block  
diagonalisation.


Having a minimised the change in the CASSCF orbitals (and hence  
maximising the change in CI coefficients) DDR is used to find the  
unitary transformation that minimises the change in the CI  
coefficients.  How is this achieved?

A.	 minimising
		|< \phi_i(q')| \phi_i(q) >|^2+|< \phi_j(q')| \phi_j(q) >|^2
	for each pair of active orbitals.

B. 	or from the overlap matrix S = <\phi_i(q)|\phi_j(q)> as
		 S(S^\dag S)^{-1/2}

C. 	or something else

The examples then mention a first order correction for the change in  
the orbitals.  What is this and how is it calculated?


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