MICROS — Generates only microstates with spin = * n*.

MICROS=`n`

For a set of spin-adapted microstates which is degenerate in both spin multiplicity and
energy, only a single member of the set is included in the expansion of the CI eigenstates. By
default, the spin adapted microstate with the smallest non-negative S_{z}
eigenvalue is chosen, i.e., S_{z} ≡ 0 for systems with an even number of
electrons and S_{z} = 1/2 for systems with an odd number of electrons.
This behavior can be overridden using this keyword, where the value of
S_{z} used will be shifted by n from the default value. For example, for
even-electron systems with MICROS=1 specified, all of the spin adapted microstates used will
have S_{z} = 1, i.e. two more alpha SOs occupied than beta SOs. For
odd-electron systems with MICROS=1 specified, the spin adapted microstates used will have
S_{z} = 3/2, i.e., three more alpha SOs occupied than beta SOs. Of
course, the value of S_{z} desired should not exceed the total spin
corresponding to the specified spin multiplicity keyword. For example, specifying
DOUBLET together with MICROS=1 will not
work because a doublet eigenstate cannot be expanded in terms of quartet microstates. For the
same reason, if QUARTET is specified
along with MICROS=1 and
CISTATE=* n* with n > 1, no doublet states will be
generated.

See Chapter 11, *Configuration Interaction* for a more complete discussion of Ampac’s CI
capability.

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