MICROS — Generates only microstates with spin =
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 Sz
eigenvalue is chosen, i.e., Sz ≡ 0 for systems with an even number of
electrons and Sz = 1/2 for systems with an odd number of electrons.
This behavior can be overridden using this keyword, where the value of
Sz 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 Sz = 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
Sz = 3/2, i.e., three more alpha SOs occupied than beta SOs. Of
course, the value of Sz 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
n with n > 1, no doublet states will be
See Chapter 11, Configuration Interaction for a more complete discussion of Ampac’s CI capability.
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