Dear Brajendra,

it depends on the spin state of the complexes which you calculate. If you have a singlet complex (multiplicity "1" in the input file) then in the output you will get only alpha orbitals, this is a default in Guassian software to for molecular orbitals of the singlet state. If you have other multiplicities, for instance, doublet, triplet, etc., then in the output you will have both alpha and beta orbitals.

Hope this helps.

Best regards,

Aleksey.

Dear Brajendra,

What you have observed is correct for all molecular quantum methods: ab initio, DFT and semi-empirical (G03, G09 and etc.)

In addition, to what Aleksey wrote, please read the following text:

Electrons can have one of two spins alpha (ms = +1/2) or beta (ms = -1/2). Spinning electrons will interact with a magnetic field (in opposite ways). When two electrons are paired in an orbital the magnetic effects are cancelled. Atoms or molecules that contain unpaired electrons are weakly attracted to magnetic fields, and are said to be paramagnetic. Atoms or molecules in which all the electrons are paired are not attracted to magnetic fields, and are said to be diamagnetic. 

Multiplicity is a property that describes an atom’s or molecule’s electronic structure. The total number of alpha and beta electrons in an atom/molecule are needed to calculate multiplicity.

Before calculating multiplicity you first calculate total spin (S).

S = (# of alpha electrons)(+1/2) + (# of beta electrons)(-1/2) 

Multiplicity = |2S| + 1 

Molecules that contain only paired electrons would have a total spin of 0 (all the alpha electrons cancel all of the beta electrons), and a multiplicity of one. A multiplicity of 1 is referred to as a singlet, two is a doublet, etc. Since a pair of electrons in an orbital will cancel out, it is only necessary to determine the number of unpaired electrons. Thus any molecule with 1 unpaired electron would be a doublet, any with two would be a triplet, etc.

For more details on Molecular Orbital Theory, please read the following publication:

http://faculty.plattsburgh.edu/tom.moffett/che112/motheorylab.pdf

Hoping this will be helpful,

Rafik

dear Rafik, when a multiplicity is three, how to draw  alpha and  beta molecular  orbital respectively? 

This is equivalent to a self-selected multiple weight of 3, i.e., the UHF is used for the calculation, which can be interpreted as a more accurate method. For UHF, alpha and beta orbitals are counted separately, and the highest occupied orbitals have one or several single occupancy, depending on the highest occupied orbital energy of alpha electrons and the highest occupied energy of beta electrons, to distinguish whether the highest occupied orbitals are specifically occupied by alpha or beta electrons.@

Aleksey E. Kuznetsov, how to calculate the energy gap between LUMO and HUMO for a metal complex that has spin state as doublet? Should the energy gap between alpha LUMO and HOMO be considered or that of beta LUMO and HOMO?

Hi, I am using Gaussian 09w for my DFT studies. Currently, i am doing DFT calculation using LANL2DZ basis set for my copper(II) and zinc(II) complexes while, 6-31G+dp basis set for my ligand. The complexes turned out to have doublet spin while ligand is having singlet spin. May i know how does this spin affects our calculation? is it valid? how do i calculate the reactivity descriptors using this alpha and beta spin?

Aleksey E. Kuznetsov Rafik Karaman