[molpro-user] HOW TO 'MERGE' TO GENERATE ORBITALS FROM FRAGMENTS

[Dr. Seth Olsen]

Hi Molpro-Users,

This communique is connected to my previous communique entitled 
'Overriding geometry (and basis) reordering in molpro?' but is not meant 
to conclude that thread.  I have figured out how to perform the 
immediate task I had before me at that time - i.e. generating an orbital 
set for benzene dimer using orbitals taken from calculations on the 
individual fragments using the MERGE facility.  Unfortunately, MERGE is 
not terribly well documented on the molpro website and the examples 
presented are not terribly useful.  This is because in these examples, 
the elements represented by each fragment don't overlap.  This means 
that they will not run into MolPro's (somewhat frustrating) habit of 
reordering the atoms and the basis set into blocks of a common element.  
This in turn means that the composite orbitals are easily generated via 
the MOVE command, and offsets, etc. are not needed.  However, if the 
atoms are rearranged, generating the appropriate MERGE deck quickly 
becomes more complicated.  Of course, for the case of benzene dimer, 
where the fragments are identical, one would run into this problem. 

In order to provide a better example of how MERGE should be used when 
fragments share common elements, I am posting a gzipped version of my 
calculation on benzene dimer to the list.  The input deck is echoed in 
the output file.  The aim of the calculation was to create an orbital 
set for benzene excimer that did not explicitly make use of symmetry, 
but where the six pi orbitals of each fragment were coupled high-spin in 
an ROHF calculation. These orbitals are then interleaved to create the 
dimer orbitals.  No final ROHF-SCF is performed on the dimer, but the 
final orbitals are output in molden and ascii format.  Note - unless you 
tweak the molden file it will not display properly using programs like 
MOLEKEL or GABEDIT.  In order to make these work, you have to reorder 
the atoms to the element-block format found in molpro (i.e. all C's 
followed by all H's).  Alternatively, one could reorder the basis and 
all the orbitals, but this is a little more of a bother.

Enjoy, and I hope it is of use to someone.

Cheers,

Seth
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