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 Kwaldner Member Profile Send PM
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 9:40:56 PM PDT - Thu, Jun 7th 2018 I have a system which I am trying to run a broken symmetry calculation. The two fragments are antiferromagnetically-coupled. The first fragment has charge -1 and S=9/2. The second fragment has charge -1 and also S=9/2. The overall molecule has charge -2 and S=0. When my input specifies the first fragment with charge= -1 multiplicity= -10, second fragment with charge=-1 multiplicity= 10, and overall molecule with charge= -2 multiplicity= 1, I do not get any error. However, when my input specifies the first fragment with charge= -1 multiplicity= -8, second fragment with charge=-1 multiplicity= 10, and overall molecule with charge= -2 multiplicity= 1, I get an error. Does anyone know why this is so? I thought multiplicity is 2S+1, and so if I treat the first fragment as spin up and second fragment as spin down, the second scenario should work and not the first? Thanks so much for any help! Edited On 7:02:46 AM PDT - Fri, Jun 8th 2018 by Kwaldner

 Sean bureaucrat Profile Send PM
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 4:35:49 AM PDT - Fri, Jun 8th 2018 Multiplicity is 2S+1 First scenario: Fragment 1; charge = -1, multiplicity = -10 so 9 unpaired beta electrons [2(9/2)+1 = 10] Fragment 2; charge = -1, multiplicity = 10 so 9 unpaired alpha electrons [2(9/2)+1 = 10] Total should then be charge = -2 and multiplicity = 1 since we have even number of alpha and beta electrons [2(0/2)+1 = 1] You specified charge = -2 and multiplicity = 1 for the total, so the calculation works Second scenario: Fragment 1, -1 charge, multiplicity = 8 so 7 unpaired alpha electrons [2(7/2)+1 = 8] Fragment 2, -1 charge, multiplicity = 10 so 9 unpaired alpha electrons [2(9/2)+1 = 10] Total should then be -2 charge and multiplicity = 17 since we have 16 unpaired alpha electrons [2(16/2)+1 = 17] You specified charge = -2 and multiplicity = 1 for the total, so the calculation fails (Even if you had a typo in your post and meant to write multiplicity = -8 for the first fragment, the total would still not be multiplicity = 1 since you would have two more alpha electrons than beta electrons, [2(2/2)+1 = 3])

 Kwaldner Member Profile Send PM
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 5:19:08 AM PDT - Fri, Jun 8th 2018 Many thanks for the help! I am still struggling with how 9 unpaired beta electrons can give a negative multiplicity of -10, and not -8, since 2(-9/2)+1=-8 ? Thanks so much!

 Sean bureaucrat Profile Send PM
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 11:50:26 AM PDT - Fri, Jun 8th 2018 The negative is just a computational flag to indicate that you want the extra electrons to be beta electrons not alpha electrons. Edited On 12:05:02 PM PDT - Fri, Jun 8th 2018 by Sean

 Kwaldner Member Profile Send PM
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 12:09:18 PM PDT - Fri, Jun 8th 2018 That is helpful, and now makes a lot of sense. Thanks so much!

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