Subject: Gaussian98 "alter" keyword

From: Elena Molteni <[email-masked]>
Date: Mon, 17 Feb 2003 17:18:36 +0100

Dear all,
I need to perform a single point energy calculation on a system containing 
Fe(II) in high spin state (S=2) coordinated to two oxygens of 
one  carboxylate and 4 nitrogens belonging to 4 histidine residues.
I had really hard times in obtaining convergence both using DFT, UHF 
methods. I use Gaussian 98.
I tried the followings:
reducing the system to the miminum number of atoms (cutting the ligands and 
saturating valences with hydrogens).
My aim was to use this output as a starting checkpoint file for the 
extensive calculation.
Unfortunately the results I obtain even with the small system are hardly 
believable. The frontier orbitals do not resemble d orbitals very much and 
the spin density is not concentrated on iron (only 1/4 of it is on Fe).
Moreover cutting the histidines is not very beautiful because i loose the 
aromaticity and this would change electronic environment a lot.
I also tried the QC method to allow the big system to converge but the 
energy continues to jump a lot.
I was told that i could take the output of a calculation converged with the 
wrong unpaired electron population and artificially put the population of 
the level with the unpaired electron into one level which is basically 
formed by d orbitals.
That is artificially put unpaired electrons onto d orbitals.
If I understood well this option is called alter.
Does anybody know how to solve this problem and if this is the way i can 
perform my calculation?

thanks a lot
Elena Molteni
e-mail:[email-masked]