Spin contamination for calculations including iron

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This is one of my input files:
start
title "name"
geometry
 load ironoxide.xyz
end
basis
 Fe  library ahlrichs_vtz
 H   library 6-31G*
 C   library 6-31G*
 N   library 6-31G*
 O   library 6-31G*
 F   library 6-31G*
 S   library 6-31G*
 Cl  library 6-31G*
end
driver
 xyz geo
 maxiter 1000
end
geometry adjust
 zcoord
 end
end
charge -3
dft
 xc b3lyp
 iterations 1000
 mult 2
 vectors atomic output fe.mo
end
task dft optimize
unset dft
charge -1
dft
 xc b3lyp
 mult 2
 iterations 1000
 vectors fe.mo
end
task dft optimize


ironoxide.xyz
     2
 geometry
 Fe                    0.00000000     0.00000000     0.38403467
 O                     0.00000000     0.00000000    -1.24308515


I get the least spin contamination after optimizing with charge -3 and changing to charge -1. However, the spin contamination increases during optimization and slight distortions in the input geometry (~0.01 Ă…) changes increases the spin contamination even more.
     Expectation value of S2:
      <S2> =      1.7552 (Exact =     0.7500)
     Expectation value of S2:
      <S2> =      1.7562 (Exact =     0.7500)
     Expectation value of S2:
      <S2> =      1.7568 (Exact =     0.7500)
     Expectation value of S2:
      <S2> =      1.7572 (Exact =     0.7500)
     Expectation value of S2:
      <S2> =      1.7573 (Exact =     0.7500)
     Expectation value of S2:
      <S2> =      1.7573 (Exact =     0.7500)
     Expectation value of S2:
      <S2> =      1.2067 (Exact =     0.7500)
     Expectation value of S2:
      <S2> =      0.7767 (Exact =     0.7500)
     Expectation value of S2:
      <S2> =      0.8908 (Exact =     0.7500)
     Expectation value of S2:
      <S2> =      1.1086 (Exact =     0.7500)
     Expectation value of S2:
      <S2> =      1.2432 (Exact =     0.7500)
     Expectation value of S2:
      <S2> =      1.2975 (Exact =     0.7500)
     Expectation value of S2:
      <S2> =      1.3333 (Exact =     0.7500)
     Expectation value of S2:
      <S2> =      1.3400 (Exact =     0.7500)
     Expectation value of S2:
      <S2> =      1.3407 (Exact =     0.7500)
     Expectation value of S2:
      <S2> =      1.3408 (Exact =     0.7500)


Are there any suggestions on how I can decrease the spin contamination?
Edited On 8:26:59 AM PDT - Tue, Jun 21st 2016 by Rasmus


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