[CHM 331] MO Calculations...

[Andrew J. Pounds]

Class -- one thing on my agenda this morning that I did not get to was 
what happens when you have a system with unpaired electrons (like O2).  
Guassian, by default, in situations where you have one or more unpaired 
electrons Gaussian does an UNRESTRICTED calculation and  treats all of 
the SPIN UP electrons in one space (normally called alpha) and all of 
the SPIN DOWN electrons in another space (often called beta).  While 
this generally gives good energies, it can really screw up your orbital 
correlation energy diagrams because the alpha and beta electrons, that 
occupy the SAME molecular orbital, are not constrained to be at the same 
energy.

To get around this problem, if you are looking at molecular orbitals for 
any system that has a spin multiplicity greater than 1, do a RESTRICTED 
OPEN HARTREE FOCK (ROHF) calculation.  This forces the alpha and beta 
electrons to utilize orbitals of the same energy.


-- 
Andrew J. Pounds, Ph.D.  (pounds at theochem.mercer.edu)
Associate Professor of Chemistry and Computer Science
Mercer University,  Macon, GA 31207   (478) 301-5627