Keyword category:

Configuration Interaction / Spin State

MICROS=n

Description

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 10, Configuration Interaction for a more complete discussion of Ampac's CI capability.

See also:

C.I., SZ.