CCL: Choice of multiplicity for NiO

 Sent to CCL by: Grigoriy Zhurko []
 I compute Ni4O4, Ni9O4 clusters and their complexes with carbon monoxide (me
 research is devoted to CO absorption on NiO). These systems can be computed in
 different multiplicities. Two approaches can be used for choosing the
 1) Compute each multiplicity and find the one with minimal energy;
 2) In the work of Thomas Bredow, only the multiplicity 19 was chosen for Ni9O9.
 He writes in his paper:
 The Ni9O9 cluster was calculated with a multiplicity of 19. This
 corresponds to a ferromagnetic state of NiO where each nickel
 is formally Ni2+ with a 3B2 ground state (local C4V symmetry).
 The ground state of solid NiO is antiferromagnetic AF2 with
 parallel spins along the (111) planes,21 but the energetic
 difference between the ferromagnetic and the antiferromagnetic
 state is within a few kJ/mol, and it is assumed that magnetic
 coupling between the Ni atoms does not play an important role
 in the CO adsorption process.
 What approach is better?
 If the approach of Thomas Bredow is more appropriate, what multiplicity should
 be chosen as the only one for Ni4O4 molecules and their complexes with CO? I
 suppose, this should be the 9 multiplicity.
 Grigoriy Zhurko